A latent variable model of developmental instability in relation to men's sexual behaviour.

A single trait's fluctuating asymmetry (FA) is expected to be a poor measure of developmental instability. Hence, studies that examine associations between FA and outcomes expected to covary with developmental instability often have little power in detecting meaningful relationships. One way of increasing the power of detecting relationships between developmental instability and outcomes is through the use of multiple traits' FA. The way multiple traits have typically been used is in trait aggregates. Here, we illustrate another way of examining relationships with developmental instability using multiple traits' FA: through structural equation modelling. Covariances between measures of FA and an outcome variable are interpreted within the context of an explicit model of associations between variables, which is tested for fit and the parameters specified within the model are estimated. We used nine traits' FA as markers of a latent variable of men's developmental instability, which was associated with the number of sexual partners. The results indicate a sizeable correlation between developmental instability and men's sexual history, despite small correlations between individual traits' FA and sexual history.

Adaptive design, female mate preferences, and shifts across the menstrual cycle.

This paper has two aims: first, to review work addressing the functional significance of variation in sexuality across the women's menstrual cycle and its implications for an understanding of human sexual nature; second, to illustrate the more general use of adaptationism in sex research. Adaptationism provides a method for recognizing adaptations, traits that evolved because they bestowed reproductive advantages upon their owners. The telltale sign of adaptation is its special design for a particular function. In recent years, evolutionary psychologists have explored changes in women's sexuality and standards of male attractiveness across the menstrual cycle. Evidence provisionally supports the idea that these changes constitute special design for the function of obtaining genetic benefits through mating with men other than primary partners.

Adaptive design, selective history, and women's sexual motivations.

I have discussed an ongoing program of research designed to examine the nature of specific sexual selection pressures that have played a role in the evolution of human mating. I have suggested that the evidence tentatively be interpreted as consistent with the existence of female preferences for traits that indicate good genes. Nonetheless, more research is needed before definitive conclusions can be reached. At the outset, I discussed several points about the application of adaptationism to an understanding of human behavior. First, special-design arguments are often critical to evolutionary explanations. Special design is not only evidence that natural selection has been at work, it can also reveal the nature of selection pressures that have shaped the organism. Second, special design often cannot be readily "read off" the observed phenotype. A convincing special-design argument may require a coordinated, coherent explanation (a nomological network; Cronbach & Meehl, 1955) of multiple and varied observations. Third, observations that are "strange coincidences" if one's explanation is not correct, but expected if one's explanation is correct, are particularly informative pieces of evidence. The research on sexual selection illustrates these points. We have attempted to provide evidence for special design in females for preferring men who demonstrate developmental precision for the benefit of obtaining good genes. We can point to no one piece of evidence that directly demonstrates this special design. The explanation that women possess special design for preferring men who possess good genes in certain mating contexts (such as extra-pair sex), however, does provide a coherent account of a wide range of findings. At least one of these findings--that women prefer the scent of symmetrical men only when the probability of conception is high--was expected by this explanation, but has no obvious alternate explanation and hence appears to be a strange, peculiar coincidence if this explanation is wrong. By no means do we have a full and complete story here. The nomological network can and should be expanded in a number of ways and, thereby, the argument for special design tightened. Whether or not the special-design argument ultimately holds up to further scrutiny, however, I would like to think that our efforts to establish it have importantly contributed to an understanding of human mating.

Self-monitoring: appraisal and reappraisal.

Theory and research on self-monitoring have accumulated into a sizable literature on the impact of variation in the extent to which people cultivate public appearances in diverse domains of social functioning. Yet self-monitoring and its measure, the Self-Monitoring Scale, are surrounded by controversy generated by conflicting answers to the critical question, Is self-monitoring a unitary phenomenon? A primary source of answers to this question has been largely neglected--the Self-Monitoring Scale's relations with external criteria. We propose a quantitative method to examine the self-monitoring literature and thereby address major issues of the controversy. Application of this method reveals that, with important exceptions, a wide range of external criteria tap a dimension directly measured by the Self-Monitoring Scale. We discuss what this appraisal reveals about with self-monitoring is and is not.

Developmental instability predicts individual variation in verbal memory skill after caffeine ingestion.

OBJECTIVE: To determine the mediating effects of developmental instability on individual differences in response to caffeine. BACKGROUND: Individual variation of drug effects might reflect broad genomic factors as well as the direct effects of specific alleles. The current study tested the hypothesis that individual differences in developmental instability, in part determined by genomic characteristics, would predict individual variation in the magnitude of caffeine-induced verbal memory deficits. Minor physical anomalies and fluctuating asymmetry were used as measures of developmental instability. METHOD: One hundred participants were (1) administered one version of the Rey Auditory Verbal Learning Test; (2) given a dose of caffeine determined by body weight (3 mg/kg); (3) assessed for minor physical anomalies and fluctuating asymmetry; and (4) given an alternate randomized version of the Rey Auditory Verbal Learning Test. RESULTS: Consistent with predictions, a composite measure of developmental instability predicted the magnitude of caffeine-induced memory decrements. CONCLUSIONS: These results may have important implications for the genetic underpinnings of individual differences in drug effects.

Taxometric analyses of sexual orientation and gender identity.

Taxa are nonarbitrary classes whose existence is an empirical question and not a matter of mere semantic convenience. Taxometric procedures detect whether numerical relations between purported indicators of conjectured taxa bear the hallmarks of true taxa. On the basis of theoretical considerations, the current study tested whether taxa underlie sexual orientation and related measures of gender identity. Two taxometric procedures, maximum covariance, making hits maximum (MAXCOV) and mean above minus below a cut (MAMBAC), were applied to Kinsey Scales and measures of childhood gender nonconformity and adult gender identity in a sample of nearly 5,000 members of the Australian Twin Registry. Results suggest that latent taxa underlie these measures. About 12-15% of men and 5-10% of women belong to latent taxa associated with homosexual preference. These percentages are greater than those of individuals who report homosexual preference, however, and hence it appears that an appreciable proportion of individuals in these taxa have heterosexual preference. An understanding of the origins of these latent taxa may be important to understanding the development of sexual orientation and gender identity.

Human sexual selection, good genes, and special design.

Evolutionary psychology seeks to understand the functional design underlying psychological processes and behavior. Theories of selection pressures developed within evolutionary biology are key components of this understanding. Because past selection pressures responsible for current design cannot be directly observed, theoretical understandings of the psychological processes and behavior must be inferred. The most important epistemological concept within evolutionary psychology is that of special design-evidence that a feature exhibits specificity, efficiency, and economy for producing a particular beneficial effect. A variety of sexual-selection processes have been proposed to account for aspects of human mating. These processes are not mutually exclusive. More than one may account for aspects of human mating. A core task of evolutionary psychology within this domain is to identify which processes account for which phenomena. I have attempted to illustrate how the search for special design is central to this endeavor.

Hedonic capacity and schizotypy revisited: a taxometric analysis of social anhedonia.

P. E. Meehl (1962) originally conjectured that hedonic capacity was an indicator of the latent class or taxon of schizotypy. However, P. E. Meehl (1989, 1990) subsequently diminished the role of hedonic capacity in his theory, indicating that hypohedonia is one of a dozen normal-range (nontaxonic) individual-differences factors that may potentiate the expression of schizophrenia. This dimensional-only view of hedonic capacity was tested by applying taxometric procedures to the Revised Social Anhedonia Scale (RSAS; M. L. Eckblad, L. J. Chapman, J. P. Chapman, & M. Mishlove, 1982) in a sample of college students (N = 1,526). Analyses indicated that the construct measured by the RSAS is taxonic in nature with a base rate approximating .10. These data are interpreted in the context of other findings suggesting that social anhedonia is an indicator of schizotypy.

The evolution of human mating: trade-offs and strategic pluralism.

During human evolutionary history, there were "trade-offs" between expending time and energy on child-rearing and mating, so both men and women evolved conditional mating strategies guided by cues signaling the circumstances. Many short-term matings might be successful for some men; others might try to find and keep a single mate, investing their effort in rearing her offspring. Recent evidence suggests that men with features signaling genetic benefits to offspring should be preferred by women as short-term mates, but there are trade-offs between a mate's genetic fitness and his willingness to help in child-rearing. It is these circumstances and the cues that signal them that underlie the variation in short- and long-term mating strategies between and within the sexes.

Sex differences in asymmetrically perceiving the intensity of facial expressions.

Emotional facial expressions are often asymmetrical, with the left half of the face typically displaying the stronger affective intensity cues. During facial perception, however, most right-handed individuals are biased toward facial affect cues projecting to their own left visual hemifield. Consequently, mirror-reversed faces are typically rated as more emotionally intense than when presented normally. Mirror-reversal permits the most intense side of the expresser's face to project to the visual hemifield biased for processing facial affect cues. This study replicated the mirror-reversal effect in 21 men and 49 women (aged 18-52 yr.) using a videotaped free viewing presentation but also showed the effect of facial orientation is moderated by the sex of the perceiver. The mirror-reversal effect was significant only for men but not for women, suggesting possible sex differences in cerebral organization of systems for facial perception.

The evolutionary genetic underpinnings of schizophrenia: the developmental instability model.

The importance of genes in the etiology of schizophrenia is well known, but the manner in which the relevant genomic factors influence neural development and the nature of selection forces operating on these factors are poorly understood. In several prominent papers, Crow has provided a unique and comprehensive theory that attempts to deal with these issues. A central aspect of his theory is that a single gene leads to reduced cerebral lateralization, increased ventricular size, and risk for developing schizophrenia. He relies greatly on Annett's right shift theory of individual variation in handedness. An alternative approach, based on the construct of developmental instability, provides a different way to conceptualize genetic influences, selection forces, and atypical lateralization in schizophrenia. We suggest that the developmental instability model has stronger empirical support and is better grounded in contemporary evolutionary genetics.

Facial attractiveness, symmetry and cues of good genes.

Cues of phenotypic condition should be among those used by women in their choice of mates. One marker of better phenotypic condition is thought to be symmetrical bilateral body and facial features. However, it is not clear whether women use symmetry as the primary cue in assessing the phenotypic quality of potential mates or whether symmetry is correlated with other facial markers affecting physical attractiveness. Using photographs of men's faces, for which facial symmetry had been measured, we found a relationship between women's attractiveness ratings of these faces and symmetry, but the subjects could not rate facial symmetry accurately. Moreover, the relationship between facial attractiveness and symmetry was still observed, even when symmetry cues were removed by presenting only the left or right half of faces. These results suggest that attractive features other than symmetry can be used to assess phenotypic condition. We identified one such cue, facial masculinity (cheek-bone prominence and a relatively longer lower face), which was related to both symmetry and full- and half-face attractiveness.

Fluctuating asymmetry, sociosexuality, and intrasexual competitive tactics.

Heterosexual men and women were told they were competing with another same-sex individual for a date with an attractive opposite-sex interviewer. After answering 6 questions, participants were asked to tell the competitor why the interviewer should choose them over the competitor. Participants' videotaped behavior was coded for different behavioral tactics. Men who were more symmetrical and who had a more unrestricted sociosexual orientation were more likely to use direct competition tactics than were less symmetrical and restricted men. Restricted men accentuated their positive personal qualities, presenting themselves as "nice guys." Structural equation modeling revealed that fluctuating asymmetry (FA) was directly associated with the use of direct competition tactics. However, the link between FA and presenting oneself as a nice guy was mediated through sociosexuality. No effects were found for women.

Menstrual cycle variation in women's preferences for the scent of symmetrical men.

Evidence suggests that female sexual preferences change across the menstrual cycle. Women's extra-pair copulations tend to occur in their most fertile period, whereas their intra-pair copulations tend to be more evenly spread out across the cycle. This pattern is consistent with women preferentially seeking men who evidence phenotypic markers of genetic benefits just before and during ovulation. This study examined whether women's olfactory preferences for men's scent would tend to favour the scent of more symmetrical men, most notably during the women's fertile period. College women sniffed and rated the attractiveness of the scent of 41 T-shirts worn over a period of two nights by different men. Results indicated that normally cycling (non-pill using) women near the peak fertility of their cycle tended to prefer the scent of shirts worn by symmetrical men. Normally ovulating women at low fertility within their cycle, and women using a contraceptive pill, showed no significant preference for either symmetrical or asymmetrical men's scent. A separate analysis revealed that, within the set of normally cycling women, individual women's preference for symmetry correlated with their probability of conception, given the actuarial value associated with the day of the cycle they reported at the time they smelled the shirts. Potential sexual selection processes and proximate mechanisms accounting for these findings are discussed.

Developmental stability and human violence.

Developmental stability (the precision with which genotypes are translated into phenotypes under physically stressful developmental conditions), is a major source of phenotypic and behavioural variation, yet researchers have largely ignored its potential role in the ontogeny of individual propensities toward human aggression and violence. In this study, we measured fluctuating asymmetry of the body and administered aggression and fighting history questionnaires to 229 college students (139 female and 90 male undergraduates). Among males, but not females, fluctuating asymmetry correlated negatively and significantly with the participants' number of fights and propensity to escalate agonistic encounters to physical violence. Principal components analyses and scree tests suggested that two psychometric factors underlie observed correlations between self-report measures of aggressive tendencies. The first factor, 'aggressive negative affect', reflected verbal aggression and hostility toward others, while the second factor, 'self-assessed fighting ability', reflected physical violence and a tendency to win fights. The two factors correlated minimally. For both males and females, the second factor correlated with number of fights while the first factor did not. Fluctuating asymmetry did not significantly correlate with either factor for either sex, but for both sexes, psychometric intelligence (IQ) correlated positively with the first factor.

Developmental instability and cerebral lateralization.

On the basis of prior studies of handedness, it was predicted that variations from modal asymmetry scores on cognitive tasks, in either direction from the mean, would be associated with an elevated incidence of classic markers of developmental instability (minor physical anomalies and fluctuating anatomic asymmetries). University students (N = 146) were administered 4 tasks that typically reveal functional asymmetries: the fused rhymed words dichotic listening task, the line bisection task, the chimeric faces task, and the cartoon faces task. A composite measure of developmental instability was computed from minor physical anomalies and fluctuating asymmetries. Participants with greater evidence of developmental instability had more atypical lateralization scores, deviating more from the sample mean, in either direction. Directional asymmetries were unrelated to developmental instability. These results suggest that developmental instability influences variation in the lateralization of cognitive skills as well as handedness.

Fluctuating asymmetry and psychometric intelligence.

Little is known about the genetic nature of human psychometric intelligence (IQ), but it is widely assumed that IQ's heritability is at loci for intelligence per se. We present evidence consistent with a hypothesis that interindividual IQ differences are partly due to heritable vulnerabilities to environmental sources of developmental stress, an indirect genetic mechanism for the heritability of IQ. Using fluctuating asymmetry (FA) of the body (the asymmetry resulting from errors in the development of normally symmetrical bilateral traits under stressful conditions), we estimated the relative developmental instability of 112 undergraduates and administered to them Cattell's culture fair intelligence test (CFIT). A subsequent replication on 128 students was performed. In both samples, FA correlated negatively and significantly with CFIT scores. We propose two non-mutually exclusive physiological explanations for this correlation. First, external body FA may correlate negatively with the developmental integrity of the brain. Second, individual energy budget allocations and/or low metabolic efficiency in high-FA individuals may lower IQ scores. We review the data on IQ in light of our findings and conclude that improving developmental quality may increase average IQ in future generations.

Evolutionary psychology and genetic variation: non-adaptive, fitness-related and adaptive.

Behavioural variation across individuals can be substantial. A broad generalization emerging from three decades of behavioural genetic studies is that most psychological individual differences have moderate broad heritabilities (30-60%). There are at least three possible scenarios for this genetic variation. First, it may be adaptively neutral and not subject to selection. Second, it may be related to fitness despite selection. Third, it may be maintained by selection for alternative adaptations. Some authors favour the first of these possibilities, but the latter two cannot be ruled out. First, temporally varying selection pressures (e.g. pathogens) can maintain fitness-related genetic variance in a population despite current selection pressures. Moreover, direct and indirect evidence on humans support the notion that some phenotypic variance is fitness related. Second, while adaptive alternatives are unlikely to be found at a level of highly complex design, frequency dependent selection can maintain variation at finer, quantitative levels. One potential example is discussed. Because of their particular relevance to evolutionary psychology, fitness-related and adaptive genetic variance deserve further attention.

Behavioral genetic variation, adaptation and maladaptation: an evolutionary perspective.

Evolutionary psychology seeks to understand the adaptive, evolved nature of humans by considering the forces of natural selection that gave rise to it. Individual humans also exhibit maladaptation, however, and some of the variation in maladaptive conditions is heritable. An evolutionary perspective can shed light on these phenomena too. Recent work suggests that developmental imprecision importantly affects the fitness of organisms and some of the genetic influences have been identified. Both theory and evidence that developmental imprecision underlies human cognitive maladaptation has begun to accumulate, although the manner in which the developing brain is affected remains largely unknown.

Brain abnormalities in schizophrenia-spectrum children: implications for a neurodevelopmental perspective.

Children with symptoms of schizophrenia-spectrum disorder (N = 20) were compared to controls (N = 20) matched for age and socioeconomic status. Structural brain abnormalities were assessed with magnetic resonance imaging and functional brain abnormalities with neuropsychological tests. Children with schizophrenia-spectrum disorder had smaller amygdala and temporal cortex volumes, along with reduced callosal areas and an unusual pattern of neuroanatomic asymmetries. No differences were noted in overall brain volume, ventricular volume, hippocampal volume, or frontal area. Schizophrenia-spectrum children were also characterized by deficits in all neuropsychological functions examined. Some types of verbal memory and frontal lobe skills were especially deficient. These results support the hypothesis that children with schizophrenia-spectrum disorder have significant brain abnormalities, similar in some ways to those seen in adult schizophrenics. In conjunction with recent primate studies, the current results draw attention to the role of the amygdala as one relevant factor in the pathogenesis of schizophrenia.