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EP is a meta-theory and integrates different results from many fields. The result is that it is working two levels of application higher then an applied psychology discipline.

In its broadest sense, there are two complementary ways to explain behavior (cf. Tinbergen):

-       Proximate explanations look at behaviour in terms of events in the immediate, proximal environment (“how” questions)

-       An ultimate explanation on the other hand refers to the evolutionary significance or function of the behaviour, a “why” question.

 

Het menselijk cognitief systeem is ontworpen door evolutie in een bepaalde omgeving (cognitieve nis).

We vertonen nog steeds gedrag dat adaptief was in de omgeving waarin de mens ontstond (Omgeving van Evolutionaire Adaptiviteit: OEA).

Kennis van evolutie en de OEA is handig in het begrijpen van menselijk gedrag.

Ouderlijke investering theorie (Trivers, 1972; 1996): de investering die ouders doen in hun nakomelingen verhoogt de fitness van de nakomelingen, maar vermindert de mogelijkheden van de ouders om te investeren in het maken van meer nakomelingen. Er is een optimum tussen voortplantingsinvestering en ouderlijke investering.

Seksedifferentiatie: het geslacht dat de kleinste aanvankelijke investering doet, meestal de man, doet de minste ouderlijke investering, maar de meeste voortplantingsinvestering.

De voortplantingssnelheid van het laag investerende geslacht is afhankelijk van de beschikbaarheid van voortplantingspartners: ze zijn voortplantingspecialisten geworden.

Het hoog investerende geslacht (gewoonlijk de vrouwen) zijn opvoeding specialisten geworden, en hun reproductie snelheid is meer afhankelijk van de beschikbaarheid van hulpbronnen, en minder van de beschikbaarheid van geslachtspartners.

There are two different selection processes: natural selection and sexual selection:

-       Natural selection is concerned with survival; it determines which individuals will survive the struggle for existence, for food, water, shelter and not failing prey to diseases and predators too soon.

-       Sexual selection determines the extent to which individuals actually reproduce themselves. It takes two forms:

a.    choosing partners (partner choice), and

b.    keeping competitors of the same sex at bay (intrasexual competition).

Investment by parents in offspring: this investment enhances the fitness of the offspring, but diminishes the opportunity for the parents to invest in other offspring. This means that there is an optimum between mating effort and parental effort.

The sexes differ in their initial parental effort; males invests the smallest sex cells and thereby make initially the least parental effort, but the most mating effort. Females makes highest (initial) investment  and are thereby forced to keep investing more than the less investing sex, simply because there is more to loose. The reproduction rate of the low investing sex (usually males) is very much dependent on the accessibility of mates: the low investing sex have become mating specialists. The other sex performs the most hatching, internal gestation, lactation, and parenting. The high investing sex (usually females) have become parenting specialists, and their reproduction rate is dependent on availability of resources, not on accessibility of mates. The low investing sex is competing for mates, while the high investing sex is choosing mates.

This difference in reproduction strategy results in “female choice, male competition”. Females will choose males that will enhance the fitness of their offspring. Males can provide protection, food, in short resources to their mates and offspring. In primates, especially in humans, not only cues of the ability to invest in offspring such as possession of resources or status, but also cues of the willingness to invest time and resources in offspring such as perceived preparedness to invest in a relationship with the mother or with her offspring, are important selection criteria (Barrett, Dunbar, & Lycett, 2002; for recent overviews of human evolutionary psychology see Buss, 1999b; Mealey, 2000; Palmer & Palmer, 2002). The other things males contribute to offspring are their genes. Females may also select mates with markers of “good genes”. Those markers can be good health, but they can also be quite “arbitrary”. Zahavi (1975) proposes that females prefer those features that indicate fitness by being actual handicaps for survival. The classical example of this is the peacock’s tail (Cronin, 1991), the sight of which made Darwin sick: in 1860, he wrote to Asa Gray, "...I remember well the time when the thought of the eye made me cold all over, but I have got over this stage of the complaint, and now small trifling particulars of structure often make me very uncomfortable. The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!"

 

Sexual Selection, Attractiveness, Symmetry and Personality

 

Sexual selection is relevant to understand sex differences in personality. In sexual selection, not only physical resources like good genes are important, but also the control of environmental resources. Especially in female mate choice, environmental resources of men are important, because this is what men can invest besides their genes. Control of resources is, almost by definition, highly correlated with the position in the social system (Betzig, 1986; Eibl-Eibesfeldt, 1989). It should not come as a surprise then, that women prefer high status men more than men prefer high status women (Buss, 1999b). This means that men have to be more concerned with acquiring and showing status if they are to be reproductively successful. They will show more dominance, i.e. behaviours that are concerned with getting, keeping and showing status than do women (Moskowitz, 1993; Pratto, 1996). Moreover, men, not much burdened by the task of reproduction, have many opportunities to play status. But not always. For instance, in sea horses the genetically males are the high investing sex. This gives us the opportunity to test hypotheses about the causes and origins of sex differences in parental behaviour.

Games. Perhaps hunting itself may be thought of as a status game: anthropological studies have shown that hunting is not a very efficient way to get calories compared to gathering (Wood & Eagly, 2002). We can expect that women will be less inclined to play status games, but instead will put more emphasis on relational aspects, showing more affiliation (Buss & Malamuth, 1996; see Taylor et al., 2000). The results of this will be gender differences in personality. The Interpersonal Circumplex (Gurtman & Pincus, 2000; Wiggins, 1979) was developed to explain individual differences in personal interaction. Its two main axes are Affiliation and Dominance (Ashton, Jackson, Helmes, & Paunonen, 1998b; Budaev, 1999; Wiggins & Trobst, 2002). Studies using the Interpersonal Circumplex find universal sex differences in Dominance and Affiliation, with men scoring higher on Dominance and Women higher on Affiliation (see Costa, Terracciano, & McCrae, 2001b; Feingold, 1994; Moskowitz, Suh, & Desaulniers, 1994). Affiliation and dominance are the ink with which human interaction is written. Men are usually physically larger, and probably because of this more verbally and physically aggressive, and they are also more engaged in status and dominance struggles, a universal finding (Eibl-Eibesfeldt, 1989; Wood & Eagly, 2002). In a comprehensive overview article concerning relational communication studied in anthropology, psychotherapy, biological approaches, semantic approaches, interpersonal evaluation research, and developmental research, Burgoon and Hale (1984) found the two most important dimensions to be dominance and affiliation. Also ethological studies of humans (Salter, 1995) and chimpanzees (De Waal,1996), as well as studies in personality and social psychology (Gifford & Hine, 1994; Gifford & O'Connor, 1987; Moskowitz, 1994; Wiggins, 1996) all find dominance and affiliation as the main dimensions of interpersonal behaviour. Studying the way men and women express affiliation and dominance in their interactions is a key factor in understanding human social

behaviour and sex differences.

Sex differences are often addressed in relation to fairness in personnel-related decision making. Fairness, i.e. making decisions on valid measurements in the sense that they are highly correlated with work outcome variables presupposes lack of bias: when instruments are biased, it is all but impossible to make fair decisions. The technical term bias refers to a distortion in statistics or measurement of test scores or ratings. Bias occurs if there are systematic group differences in score distributions or indices of validity for reasons that are unrelated to the trait under consideration. Instead, such differences are caused by interference of intervening variables like stereotypes, culture, self-esteem (e.g. Barrett, 1998; Conway, Jako, & Goodman, 1995; Guion, 1998; Hough, Oswald, & Ployhart, 2001). For instance, when attractive people are assessed more positively by assessors while attractiveness is not a criterion for job success, attractiveness bias occurs, resulting in unfair and possibly counterproductive selection procedures. Of course, when attractiveness is relevant to job success, it is perfectly rational to include it in the set of predictors. However, if attractiveness still influences the scores on presumably unrelated traits (say, intelligence), attractiveness bias still may occur. Often, the assumption is made that all group differences reflect bias in the used instruments. Of course, this is only justified if one can be sure that the difference does not reflect a real difference between the groups (Guion, 1998). Whether group differences in test scores are caused by measurement bias or not can only be settled by additional research. Research into rater bias is concerned with influences such as ethnicity, sex, cognitive variables, personality variables or attractiveness of assessors and applicants on the reliability or validity of ratings (Buttner & McEnally, 1996; Foster, Dingman, Muscolino, & Jankowski, 1996; for a model, see Landy & Farr, 1983; Walsh, Weinberg, & Fairfield, 1987; Watkins & Johnston, 2000). Evolutionary psychology can make substantial contributions to this research with regard to understanding sex differences and the role of attractiveness. Bias in standardised tests is a real possibility, but in less standardized, more subjective methods of selection, bias is even more a threat to fairness. The often used methods of ratings or interviews are much less standardized, less reliable and more subjective, but yield possibly other more useful information for selection decisions than standardised tests. Validity generalisation is a special kind of meta-analysis technique that enables researchers to combine the results of many predictive validity studies; for reviews of validity generalization, see Murphy (2003). A validity generalization study by Schmidt and Hunter (1998) showed that procedures that combine a general mental ability test with a structured interview have the highest predictive validity. Interviews and ratings also have high face validity, and so they are often used. In fact, although ability tests are often used in personnel selection, in many

selection procedures an unstructured or badly structured interview is the only selection instrument. This means rater bias caused by sex differences of raters and ratees, the attractiveness of ratees and the interaction between these variables are an important potential threat to validity and fairness. Indeed, attractiveness often turns out to be an important variable that causes bias in personnel decisions. Meta-analyses by Cesare (1996) of validity studies and one by Hosoda, Stone-Romero and Coats (2003) of experimental studies have confirmed that attractive people have more chances to get selected or promoted in the occupational domain than non-attractive people, a finding that is not confined to western societies, but is also found in a South-East Asian society (Chiu & Babcock, 2002). Cesare (1996) found more bias against non-attractive women than against non-attractive men, while Hosada et al. (2003) found no sex differences, but caution that this could be because of the general nature of their study: sex differences regarding attractiveness might be domain specific (Feingold, 1992; Feingold, 1994; Jackson, Hunter, & Hodge, 1995). Moreover, professionals appear to be as susceptible to attractiveness bias as non-professionals (Hosoda, Stone-Romero, & Coats, 2003; Marlowe, Schneider, & Nelson, 1996).

Bias in selection and promotion might be a part of the answer to the question why women are underrepresented in demanding and high status jobs. However, differences in ambition and work orientation might provide a partial answer too. Indeed, it is likely that sex differences in personality and behaviour will have an impact on work related behaviour. Women might be less inclined than men to apply for and stay in competitive and high status jobs because on average women may be less competitive and less interested in status. Evolutionary psychological research into sex differences and parental investment theory have indeed shown that cross-culturally men are more competitive, dominant and ambitious then women (for overviews, see Barrett, Dunbar, & Lycett, 2002; Buss, 1999b; Costa, Terracciano & McCrae, 2001b; Wood & Eagly, 2002). Differences in ambition and competitiveness will result in self-selection of possible job incumbents. This self-selection caused by low levels of ambition and competitiveness would diminish the pool of suitable women candidates relative to that of men of high status jobs. A recent study by Van Vianen and Fisher (2002) showed that women’s overall ambition was lower than men’s, and that organisation culture influenced women’s career choices (see also Eagly & Johnson, 1990; and Eagly, Karau, Miner, & Johnson, 1994).

There may be a biological base for sex differences in competitiveness, and related to that in ambition. Competitiveness is influenced by hormones during development and daily behaviour. Studies that related testosterone with aggressive and competitive behaviour in contact sports all found that in men higher testosterone levels were related to more competitive behaviour, and winning caused a testosterone boost after the match (Filaire, Maso, Sagnol, Lac, & Ferrand, 2001; Gonzalez-Bono, Salvador, Ricarte, Serrano, & Arnedo, 2000; Mazur, Booth, & Dabbs, 1992; Salvador, Suay, Gonzalez-Bono, & Serrano, 2003; Salvador, Simon, Suay, & Llorens, 1987; Salvador, Suay, Martinez-Sanchis, Simon, & Brain, 1999; Serrano, Salvador, Gonzalez-Bono, Sanchis, & Suay, 2000; Wagner, Flinn, & England, 2002). In women, the relation between testosterone and competitiveness is less clear. Testosterone might not be linked to a fright-fight-flight behavioural system in women, but to a tend-and-befriend system (Taylor et al., 2000). The evolutionary explanation behind this is that women in the EEA (and now) usually cannot defend themselves by fighting against males, simply because men are much stronger (see for a meta analysis Hough, Oswald, & Ployhart, 2001). Forming and maintaining coalitions are the preferred female defense. Support for this comes from a study on female judoka’s, where no testosterone boost was found after winning a match, and where the winner indeed showed more tending-andbefriending behaviour (Bateup, Booth, Shirtcliff, & Granger, 2002). In work-related behaviour, testosterone influences competitiveness and aggression, which has been demonstrated by studies using samples consisting of employees: testosterone appears to be related to competitiveness in trial lawyers, and social tendencies in fire fighters regarding the choice of their job content (Dabbs, de la Rue, & Williams, 1990; Dabbs, Alford, & Fielden, 1998; Fannin & Dabbs, 2003).

In sum, evolved sex differences in preferences regarding attractiveness causing bias in selection, and evolved behavioural sex differences in dominance and affiliation could provide a clue why men and women are found in different jobs.

Using an evolutionary perspective, we looked at sex differences in the influence of facial attractiveness of applicants in hiring decisions, and determined whether expected contact intensity with applicants and experience in personnel selection influenced these preferences. Students and rofessionals selected mock applicants who varied on attractiveness for jobs that differed on expected contact intensity. We found effects of evolved preferences regarding mate selection and intrasexual competition when the expected contact intensity was high. People showed a preference for attractive opposite-sex applicants (mate selection). Furthermore, women preferred female applicants with low attractiveness over female applicants with high attractiveness (intrasexual competition). We found no salient differences between professionals and students.

The heuristic ‘what is beautiful is good’ permeates our cognitions. Attractive people are considered to be more competent (Jackson, Hunter, & Hodge, 1995), especially socially competent (Eagly, Ashmore, Makhijani, & Longo, 1991), and they are judged more positively (Langlois et al., 2000).  A large body of evidence shows that due to this “attractiveness heuristic” attractive applicants do have an advantage in hiring decisions (Cesare, 1996; Chiu & Babcock, 2002; see Marlowe, Schneider, & Nelson, 1996), although the relation between attractiveness and competence in reality seems to be small (Langlois et al., 2000; Zebrowitz, Hall, Murphy, & Rhodes, 2002). Moreover, professionals are as susceptible to attractiveness bias as non-professionals are (Cesare, 1996; Gilmore, Beehr, & Love, 1986; Hosoda, Stone-Romero, & Coats, 2003; Marlowe, Schneider, & Nelson, 1996; Olian, Schwab, & Haberfeld, 1988). From the point of view of decision theory, attractiveness should only be a criterion in personnel selection when attractiveness has a presumed or observed influence on job success. For some high social contact jobs attractiveness might be important, but for most other jobs attractiveness will not add to job success; yet, highly attractive applicants (particularly highly attractive women) are often preferred. In our view the preference begs for a theoretical explanation. Moreover, the effects of the interaction of sex of applicants and sex of assessors with attractiveness of applicants in selection have not yet been systematically examined. Cesare (1996) found more bias against non-attractive women than against non-attractive men. Hosada et al. (2003) found no sex differences in their meta-analysis. They caution that this counterintuitive finding could be due to the general nature of their study and that sex differences in attractiveness might depend on specific constellations of assessors, applicants, and job demands (Feingold, 1992; Feingold, 1994; see also Jackson, Hunter, & Hodge, 1995). An evolutionary psychology perspective could well explain the preference for attractiveness and is able to predict when interactions between sex of assessor and sex of applicant in the importance of attractiveness will occur, by relating choices in personnel selection to preferences to mate choice and intrasexual competition. Men and women differ in the preferences they have for same-sex and opposite-sex attractiveness (Barrett, Dunbar, & Lycett, 2002; for overviews, see Buss, 1999b; Buss & Schmitt, 1993; Miller, 2000; Ridley, 1994). A universal finding is that men place more value on youth and good looks than do women. Moreover, what is a pretty face is not arbitrarily or culturally defined, but has deep evolutionary roots having to do with fertility and youth (Fink & Penton-Voak, 2002;

Johnston, 1999). For instance, in females, neotenous traits in faces (i.e., childlike features such as large eyes, small nose, and small ears) are considered to be universally more attractive (e.g., Jones & Hill, 1993). Moreover, a large number of studies have shown that symmetrical faces are often considered to be attractive (Hume & Montgomerie, 2001; Jones et al., 2001; for recent studies, see Mealey, Bridgstock, & Townsend, 1999). The relation between symmetry and attractiveness may in fact be mediated by judgement of apparent health; there is an opposite-sex bias in sensitivity to facial symmetry when judging health from photographs (Hume & Montgomerie, 2001; Jones et al., 2001; but see Kalick, Zebrowitz, Langlois, & Johnson, 1998; Milne et al., 2003; Rhodes et al., 2001). Furthermore, symmetry is related to reproductive behaviour, with symmetrical individuals having higher levels of sexual activity with different partners (for recent overviews see Gangestad & Simpson, 2000; Kowner, 2001; Mealey, Bridgstock, & Townsend, 1999). In sum, it seems that the heuristic to focus on attractiveness has some biological basis: Facial attractiveness based on facial symmetry is a visual marker for fertility, genetic quality, and health. An evolutionary perspective provides a parsimonious and powerful explanation for the findings why we find certain faces beautiful and others not, and why men and women differ in their preferences; it also puts other preferences and behaviours that have organizational consequences like hierarchical tendencies and power use in perspective (for recent overviews and discussions of the application of evolutionary approaches in the study of organizational behaviour, see Nicholson (1997; 2000; 2001), Pierce and White (1999), Sandelands (2002) and Colarelli (2003). It explains these findings, and connects them to a wealth of other findings regarding human and animal behaviour by referring just to one simple concept, derived from general evolution theory: sexual selection. In a sexual reproducing species like humans, sexual partners have to be selected and competed for: Darwin called this “sexual selection”. Those people in our evolutionary past who had a set of cognitions that enabled them to choose the right partners, had a competitive advantage in producing healthy offspring. We inherited their heuristics in that we also use cognitions that make us choose fertile partners of high genetic quality, good development and health. Facial attractiveness is a proxy for these things; people who found the right indices attractive chose partners of high quality, and because of that, they had more surviving offspring. This inherited heuristic (i.e. the set of cognitions that regulate partner choice based on (facial) attractiveness), will slightly differ between men and women because in the course of human evolution, men and women met different restrictions regarding the number of offspring they could produce. Men were restricted by sexual access to fertile women, while women were restricted by the limited capacity of their body to produce children. For men it was adaptive to choose women who displayed cues of high fertility. Because fertility, health and good development are correlated with appearance, men will tend to place higher value on “good looks” and youth than women do. Information about good looks is for a good part gathered from facial cues (Thornhill & Grammer, 1999). Therefore, men are expected to place higher value on a pretty face than women do. Women’s ability to produce offspring is not restricted by sexual access to fertile men, as most men are fertile until the day they die. Rather, women are limited by the capacity of their own bodies to produce offspring. To be adaptive, women pay more attention to the ability to invest in offspring, which is far less correlated with facial attractiveness than direct physical cues such as health and fertility. Examples of proxies for the ability to invest in offspring are status and intelligence; these characteristics are valued more by women than by men in prospective partners (see for an overview and an excellently balanced comparison study Li, Bailey, Kenrick, & Linsenmeier, 2002). Same-sex individuals compete for partners of the other sex. This is referred to as intrasexual competition. This is the mirror of partner choice: What the opposite sex likes become weapons or resources of competition within same-sex individuals. From the fact that men are more interested in physical cues in prospective partners than are women, we can predict that women dislike good looks in other women more than men dislike attractiveness in other men (for an overview on female competition, see Campbell, 2004). Men, on the other hand, will compete more regarding status and resources. That is why they will not mind attractiveness in other men as much as women mind attractiveness in other women. In three studies, we tested if these evolved preferences are still operating today in a setting

that some parallels with mate selection, namely personnel selection. We expected that the influence of evolved preferences would be discernable in modern personnel selection settings, because choosing people is exactly what these evolved preferences were meant to facilitate. Of course, sexual selection implies a certain amount of expected contact intensity. If people do not expect to have any contact with the chosen applicant, mechanisms of sexual selection and sexual competition may not be adequately triggered because there are no perceived mating opportunities. It might be the case that low expected contact intensity with applicants, as is the case in selection procedures where external experts are used, does not trigger evolved (opposite-sex) mate choice or (same-sex) intrasexual competition mechanisms.

The most influential alternative theory to evolutionary explanations using parental investment theory explaining sex differences is social structure theory (Eagly & Wood, 1999), also called the “biosocial” theory of sex differences (Wood & Eagly, 2002). In short, Eagly and Wood propose that psychological sex differences are not caused by natural or sexual selection. Instead, psychological sex differences are the consequence of socializing influences of a society that gives men and women different roles. Eagly and Wood do propose that this specific sex-role distribution, or division of labour, is rooted in biology, because men and women have different bodies that are differently suited to different tasks; this is why they call their theory “biosocial”. Essentially, biosocial theory claims that the only sex differences that are caused by evolution are physical sex differences, while psychological sex differences are the consequence of the different roles men and women are assigned to because of their bodies and competencies following from having different bodies. The first argument consists of the fact that it is highly improbable that evolution would stop causing sex differences when it comes to the brain. Why would evolution stop at bodies? Cognitions, preferences, emotions and ultimately behaviour are at least as relevant to survival and reproduction as bodies are. There is no logical reason why selection should not shape the brain and hormonal systems (see also Friedman, Bleske, & Scheyd, 2000). That an evolutionary process is capable in designing behaviour is beyond any doubt: Ethologists have shown that many species show adaptive species-typical behaviours. Logically, there is no reason why an evolutionary process would not design different psychological mechanisms in men and women when they have encountered systematically different adaptive problems, as in fact they have. Moreover, Biosocial theory does not provide an explanation why this barrier would be needed, and perhaps more important, it does not provide a mechanism for this sudden barrier that the evolutionary process presumably encounters when it comes to psychological sex differences. Secondly, biosocial theory does not provide an adequate explanation for the role of hormones in causing psychological and behavioural sex differences. According to Wood and Eagly, hormonal changes underlying sex differences are triggered by sex roles. Without any doubt it is true that situations are able to trigger hormonal responses, and is it also true that sex roles put people into different situations with different expectancies. However, this does not mean that sex roles necessarily precede hormonal reactions. On the contrary, it is much more likely that sex differences in hormonal patterns are in part the cause of psychological sex differences. The existence of non-sex-role-related hormonal differences that cause sex differences forms an argument for this. Hormonal influences during prenatal development are the most obvious example. Congenital adrenal hyperplasia (CAH) occurs when the adrenal gland produces excessive levels of androgens during development in the womb. Girls with CAH have a masculine behavioural pattern; they like to play with boys and with “boy toys”, they perform better on targeting tasks and have a higher preference for masculine occupations (for overviews, see Berenbaum, 1999; Collaer & Hines, 1995; Hines et al., 2003; Hines, Brook, & Conway, 2004; Hines & Kaufman, 1994). A related syndrome in men is androgen-insensitivity syndrome (AIS). Boys with AIS are insensitive to androgens, which causes a female–like development (Hines, Ahmed, & Hughes, 2003). Furthermore, non-clinical studies point to the importance of circulating hormones during pregnancy on sex differences later in life. The most informative regarding the claims of Biosocial theory is a large-sample longitudinal study by Hines et al. (2002), who found a linear relationship between testosterone levels during pregnancy and masculine behaviour in female children at the age of three to four, but no relationship between the children’s sex-typed behaviour and having older brothers or sisters, nor with the absence of a father (or father figure) or traditionalism of sex-role orientation of the parent(s). Biosocial theory claims that hormonal influences are triggered by sex role demands. But hormonal influences can actually precede sex-typed behaviour. Female-to-male transsexuals show increased spatial performance, an increase in aggression and sex drive after androgen therapy, which are all male features; male-to-female transsexuals show higher verbal memory performance and lower sex drive after estrogen treatment, which are all female features (Miles, Green, Sanders, & Hines, 1998; Van Goozen, Slabbekoorn, Gooren, Sanders, & Cohen-Kettenis, 2002; van Goozen, Cohen-Kettenis, Gooren, & Frijda, 1995). Another example comes from female sexual behaviour that varies with the menstrual cycle, a finding hard to reconcile with biosocial theory, but readily understood within an evolutionary framework. On peak fertility, women are more easily sexually aroused and show more interest in physical features of a potential mate (Gangestad & Cousins, 2001; Pillsworth, Haselton, & Buss, 2004; van Goozen, Wiegant, Endert, & Helmond, 1997). It is hard to see how sex roles may influence sexual behaviour specifically according to the phase of the menstrual cycle. Summarizing, the argument that sex roles can trigger hormonal reactions that are related to sex-typed behaviour is not a conclusive one, because there are numerous instances where hormonal influences actually trigger sex-typed behaviour in a way that is easily reconciled with the roles men and women have had in evolutionary history in reproduction. Thirdly, biosocial theory does not provide a satisfactory explanation for the finding that very small children and young primates show sex-typed behaviour. Biosocial theory maintains that sex differences in cognition and behaviour are the consequence of society putting men and women into different sex roles. However, as any parent knows, it is hard to get children to like opposite-sex stereotyped toys. Very young children, even babies, show preferences for sex-typed toys (Campbell, Shirley, & Candy, 2004; Campbell, Shirley, Heywood, & Crook, 2000; O'Brien & Huston, 1985). Moreover, even young primates show sex-typed preferences for toys. Vervet monkeys show sex differences in toy preferences similar to those documented previously in children. The percentage of contact time with toys typically preferred by boys (a car and a ball) was greater in male vervets than in female vervets, whereas the percentage of contact time with toys typically preferred by girls (a doll and a pot) was greater in female vervets than in male vervets (Alexander & Hines, 2002). These findings are not in favour of biosocial theory, because it is hard to imagine how society influences the preferences of babies and young primates by means of sex-role allocation. Fourthly, biosocial theory is not parsimonious and does not enhance integration with other sciences. One of the oldest and most important scientific principles is Occam’s razor: Pluralitas non est ponenda sine necessitate" - plurality should not be posited without necessity. Explanations should be as parsimonious as possible. This principle applies across sciences too. For instance, biologists should explain why flying fish drop back into the water by referring to gravity, not to biological concepts. In fact, they should not explain it all, because physics already has done so. The same applies to Biosocial theory. An evolutionary perspective makes it possible to explain and predict human sex differences by using the same concepts with which it is possible to explain, say, sex differences in nesting behaviour of birds, the shape, colour and form of a peacock’s tail and the existence of predominantly female prostitution in humans. Biosocial theory maintains that these concepts are not enough to explain psychological sex differences, but postulates extra concepts: a causal influence of sex roles and a barrier to the evolutionary process, but without giving a rationale why there is a necessity to do so.

A case in point is the often cited evidence for Biosocial theory derived from the reanalysis of the data concerning sex differences in mate choice collected by Buss in 37 countries (Buss, 1989) by Eagly and Wood (1999). A biosocial explanation of the data proposes that the pattern of sex differences found in 37 countries can be just as well explained by differences in power between the sexes, which are in turn the consequence of the different bodies of men and women. This is not the case (see Friedman, Bleske, & Scheyd, 2000; Kenrick & Li, 2000; Kleyman, 2000): it does explain why younger women prefer older men by referring to power imbalances, but it does not explain why older men prefer younger women, nor why young men, in their late teens and early twenties, prefer women who are older than they are. But even if it did provide an explanation that would fit as well as an evolutionary explanation, an evolutionary explanation still has to be preferred over the explanation of Biosocial theory, because of the application of Occam’s Razor: an evolutionary explanation is simply more parsimonious. Fifthly, biosocial theory does not explain overlap of human sex differences with animal behaviour. Biosocial theory gives social roles a causal power in the generation of sex differences. However, whywould then in different species that often lack anything even remotely resembling a culture or social learning the same sex differences as we see in humans occur?. Invariably, the high investing sex is choosier, more interested in the possession of resources in future mates, while the low investing sex is less choosy and more sensitive to cues signalling fertility in potential mates (Barrett, Dunbar, & Lycett, 2002; Darwin, 1871; McFarland, 1993; Zahavi, 1997). This is surely not the consequence of sex role allocation, because animal culture is simply not that sophisticated. However, the correspondence is too exact to be a coincidence. Neither is the “bio” component of biosocial theory, i.e. the mean difference in body size of men and women, of use to explain this overlap. For instance, why would strong or high status men prefer young women (indeed, why would men prefer young women at all?). In fact, Wood and Eagly themselves argue that body size differences between men and women are relatively small and not very suitable to explain sex differences. However, size dimorphism, even if it is small, is very likely to shunt the evolutionary process into a specific direction. For instance, it is perfectly understandable that women do not react to physical threats with a fright-flight-fight system, but with a tend-and-befriend response (Taylor et al., 2000), simply because it is not effective to fight or flight when your opponent is most often stronger and quicker than you are. I agree that it would indeed be inappropriate to interpret body dimorphism as the only ultimate cause of human psychological sex differences. But this is not what evolutionary psychology does. It is however exactly what Biosocial theory does. Lastly, biosocial theory is relatively silent about the remarkable fit between partner-choice preferences of men and women and their potential reproductive success. Universally, men and women find those things attractive in the opposite sex that were probably related to reproductive success in the Environment of Evolutionary Adaptedness. Men like fertility cues to a highly specialized degree. For instance, although male preferences for thin or more voluptuous female figures is culturally variable (Furnham, Moutafi, & Baguma, 2002; Sugiyama, 2004), the preference of a waist-to-hip ratio of 0.7 is not, because it is reliable index fertility in women (Penton-Voak et al., 2003; Singh, 1995; Streeter & McBurney, 2003). Female preferences for attractiveness in men are influenced by markers of “good genes” like symmetry and masculine faces (Fink & Penton-Voak, 2002; Grammer & Thornhill, 1994; Jones et al., 2001; Rhodes et al., 2001; Thornhill & Gangestad, 1993). Moreover, these preferences are influenced by cyclic hormonal changes within women: women express more interest in masculine faces when they are at peak fertility (Little, Penton-Voak, Burt, & Perrett, 2002). Similar cases could be made for facial and body symmetry, which are indices of developmental stability, health and good genes (Gangestad & Thornhill, 1997; Gangestad & Thornhill, 2003; Gangestad, Thornhill, & Yeo, 1994; Kowner, 1996; Kowner, 2001; Manning, Koukourakis, & Brodie, 1997; Mealey, Bridgstock, & Townsend, 1999).  Again, there are cyclic variations in the preference for symmetrical males (Gangestad & Thornhill, 2003). But perhaps the most impressive case of sex differences in human partner choice comes from immunology. Females are able to detect by assessing the smell of men if they have similar or different immune systems from themselves. Partners with different immune systems are preferred, which s evolutionary makes sense evolutionary speaking, because combining two immune systems that are as diverse as possible in a child yields maximum disease resistance (for a recent overview, see Thornhill et al., 2003). It is hard to conceive how biosocial theory would be able to explain these findings. It is impossible that allocation to sex roles based on different bodies of men and women would be able to cause these exact, adaptive sex differences in partner choice. In conclusion, biosocial theory may be a special elaboration of evolutionary psychology of human sex differences, but it is not a fully alternative explanation. Its scope and explanatory power are severely limited compared to that of evolutionary psychology. Moreover, it is for a large part incompatible with evolutionary reasoning (Friedman, Bleske, & Scheyd, 2000): the claim that evolution stops at causing sex differences in the human mind is a serious claim, for which considerable logical considerations, possible mechanisms and empirical prove is needed. Biosocial theory gives none. It does not integrate findings of psychology to those of other fields of psychology or biology. Lastly, Biosocial theory is often not able to explain empirical findings that evolutionary psychology can explain easily. However, Biosocial theory does provide an additional explanation to those of evolutionary psychology of some observed sex differences. There is no doubt that sex roles influence behaviour. Cultural and developmental influences may diminish, exaggerate or even reverse sex differences. Partner choice preferences do vary: sometimes women are referred to be chubby, and sometimes a skinnier figure is preferred, at least in art and in the media. Cultures make men and women more different or more similar, and in fact, masculinity-femininity is one of the fundamental cultural dimensions (Hofstede, 1998). Lacking however, is a formulation of how and why these cultural influence cause sex differences. There is no “invisible cultural hand” pushing men and women into arbitrarily different directions. On the contrary, sex roles make sense from an evolutionary point of view. In this sense, biosocial theory does not tell us more than we already knew. Evolutionary psychology tells us more with fewer concepts. Biosocial theory would explain the results of the studies of this dissertation in terms of social influences ultimately caused by power differences between men and women. Of course, social influences and learning are relevant in explaining gender differences. But biosocial theory does not explain why social influences and learning processes are the way they are, while making more assumptions than an evolutionary approach. The results of the studies in this dissertation can be easily related to studies in animal behaviour regarding mate choice, dominance, and status striving using an evolutionary perspective. The results of the studies described in Chapter 2, where effects of attractivenessand sexual selection were identified, would be explained by referring to social learning processes that cause men to prefer attractive women more than women to prefer attractive men. Why these sexual selection preferences would be discernable in personnel selection is understandable using a evolutionary perspective. It maintains that personnel selection and sexual selection partly trigger the same cognitive mechanisms. A similar case can be made for the results of the study described in Chapter 3, where gender differences in ambition were identified. A biosocial explanation would have to postulate that men learn to be more and women learn to be less ambitious, one way or another. Additional explanations have to be postulated here than for explaining gender differences in attractiveness: men and women do not only learn to have differences in preference for attractiveness, but also in ambition. The same applies to the study described in Chapter four: a biosocial explanation postulates that men learn to be more dominant and women learn to behave more affiliative. An evolutionary perspective uses one explanation: sexual selection and it explains the results better than a biosocial explanation, where many additional assumptions are needed.

An often-heard criticism of applying evolutionary theorizing to psychology is that it is like using a cannon to kill a fly. This argument may be applicable when it is used to criticize specific post-hoc explanations, but it is not applicable to the process of generating hypotheses or establishing vertical integration of psychological explanations with biological explanations. To deliberately limit the scope of hypothesis-generation as well as attempts to connect theories and explanations with each results in missing opportunities to establish connections with a wealth of theories and findings of other sciences or fields that might be relevant. A prediction turns out to be right or wrong, and findings are useful or not 8, while an explanation of findings post-hoc has virtually no safeguards against over-interpretation (in evolutionary psychology this is usually adaptationism) and using false assumptions. Most critiques of evolutionary psychology are centred around the alleged over-interpretation by evolutionary psychologists (see for an example in evolutionary personnel psychology Usher, 1999). Alternative explanations will always exist, but the possibility to rule out these explanations by showing that evolutionary explanations are more parsimonious and better imbedded in higher level explanations and sciences like biology or physics has to be taken very seriously. On the basis of the studies in this dissertation it cannot be concluded that the sex differences that were found must have an evolutionary genetic basis. They were not meant to do so. The aim of evolutionary personnel psychology is not to test evolution theory or to solve naturenurture debates. An evolutionary personnel psychology should be aimed at answering

psychological questions having to do with personnel-related questions. It may use evolution theory as a meta-theory, like evolutionary psychology does. This probably means there will be a large part of personnel psychology that is not evolutionary personnel psychology, but that is of course perfectly acceptable: A large part of biology is not evolutionary biology, and a large part of psychology is not evolutionary psychology. However, insights from evolutionary biology and evolutionary psychology may help to guide personnel psychology research and to make sense of the myriad of findings that are already there. Personnel psychology should be as compatible with evolutionary reasoning as it is with its main mother-discipline, personality psychology, and borrow insights from evolutionary psychology in the same way. In the second chapter, we assessed the effect of gender differences in evolved preferences for attractiveness in a job selection setting by presenting black and white photographs together with a mock personality description. It might be the case that this relative lack of information about the applicants gives artificially too much weight to attractiveness in the decision process: the assessors do not have much other information to work with. In real selection procedures, usually more information about the applicants is available, and this might diminish the role of attractiveness. This relative lack of decision information thus may influence the ecological validity of the findings. However, confirmation of the hypotheses regarding gender differences is not compromised by this, because it is highly unlikely that the

Of course, in the end, theories determine what counts as facts. On a given set of “facts” fits a unlimited amount of theories but to keep things discussable, it is necessary to leave destructive concepts like “underdetermination of theories by facts” and social constructions of scientific facts out of the discussion here. If we do not, we might as well stop doing science.

lack of additional information would influence gender differences in decision processes based on attractiveness information in a systematic way. The practical implications of this conclusion lie in avoiding the potential bias caused by evolved preferences. The use of men and women, and withholding or postponing of (most often) irrelevant information regarding the appearance of applicants are obvious solutions. The third chapter, in which gender differences in ambition were assessed, made the ecological validity of the methods to measure ambition a subject of investigation by incorporating situational limitations and time budgets. In fact, only those items with high ecological validity showed gender differences. A limitation lies in the applicability of the used methods in HRM practice: the content of the items is very sensitive to impression management. For instance, applicants will understand that it is not a good idea to indicate that they think leisure time is

much more important than overwork. Of course, this posed not many problems in the sample we used here, because they were not being assessed for anything. The finding that women report lower levels of ambition than do men when appropriate instruments are used might partially explain why in most high status positions there are many more men then women. Gender differences in ambition may work as a self-selection device in personnel selection procedures. For example, there is evidence that self-selection based on gender differences in preferences for organization culture play a role in the job choices of men and women (van Vianen et al., 2002), and it is likely that gender differences in mean level of ambition have a self-selection effect too. This self-selection on basis of gender differences in ambition causes a restriction of the range of suitable female applicants (for a theoretical analysis of the consequences of restriction of range, see Roth, Bobko, Switzer, & Dean, 2001): there are simply less suitable women to choose from, and logically, this problem gets worse in higher levels of the organizational hierarchy because of the cumulative effects of self-selection. This, in turn, may also hamper the effectivity of positive action programs. In the fourth chapter, where affiliative and dominant behaviours of men and women during an interaction with an actor during a real life assessment centre was observed, has a very high ecological validity. The price that had to paid for this was the use of a relatively small sample and a relative lack of standardisation of the observation environment. The participants were not exposed to exactly the same behaviours of the actor, but rather to a natural interaction between actor and participant took place. Lastly, the participants were a selected sample of highly educated men and women. This last limitation was actually an advantage in testing the relative strength of evolved preferences in the face of cultural and social influence, but on the other hand, it limits the generalization of the findings to other groups of people. However, because gender roles are more pronounced in lower educated groups of participants, it is likely that research using these groups of participants will actually find more pronounced gender differences.

The sub-theory of evolution theory that I used here is parental investment theory, and this has resulted in predictions of sex differences. It was chosen because sex differences in parental investment is the main theme in evolutionary psychology, as browsing through the main journals will confirm. The studies in this dissertation suggest that parental investment theory can provide insights that are useful to personnel psychology. This should not be too surprising: work behaviour is human behaviour after all. However, there is a surprising lack of studies and articles in evolutionary work psychology. The most important journal of evolutionary psychology, Evolution and Human Behaviour, never published a work-related article. The few existing publications in evolutionary work psychology have been overviews and discussions of the application of evolutionary approaches to the study of organizational behaviour, see Nicholson (1997; 2000; 2001), Pierce and White, (1999), Sandelands, (2002), but these include no explicit empirical studies. These overviews are informative and thoughtprovoking, but to establish the field of evolutionary personnel psychology with a sound base, empirical studies are needed. The only empirical study so far in evolutionary personnel psychology is concerned with parental investment theory and predictions concerning the content of letters of recommendation (Colarelli, Hechanova-Alampay, & Canali, 2002). In all studies of this dissertation, evidence was found that gender differences in preferences and behaviours that are related to different roles in reproduction of men and women, as predicted by parental investment theory, are relevant in understanding work behaviour: Men and women choose each other in personnel selection procedures in the way comparable to they would choose partners. Men behave more dominantly while women show more affiliative behaviour during a managerial assessment centre. Men indicate to be more ambitious and concerned with a high standard of living while women are more concerned with caring, when asked about their life goals and time distribution. Not only were all these gender differences predicted by parental investment theory, but an evolutionary perspective also provides a parsimonious and powerful explanation of these differences and relates these findings. It is this power to connect, explain, and predict that makes evolutionary psychology valuable for personnel psychology. There are many more evolutionary studies waiting to be applied. I hope the studies in this dissertation may provide a starting point.