Kin recognition: evidence that humans can perceive both positive and negative relatedness.

The evolution of spite entails actors imposing costs on 'negative' relatives: those who are less likely than chance to share the actor's alleles and therefore more likely to bear rival alleles. Yet, despite a considerable body of research confirming that organisms can recognize positive relatives, little research has shown that organisms can recognize negative relatives. Here, we extend previous work on human phenotype matching by introducing a cue to negative relatedness: negative self-resembling faces, which differ from an average face in the opposite direction to the way an individual's own face differs from the average. Participants made trustworthiness and attractiveness judgements of pairs of opposite-sex positive and negative self-resembling faces. Analyses revealed opposing effects of positive and negative self-resembling faces on trustworthiness and attractiveness judgements. This is the first clear evidence that humans are sensitive to negative relatedness cues, and suggests the potential for the adaptive allocation of spiteful behaviour.

Sexual orientation and handedness in men and women: a meta-analysis.

Recent findings suggest that sexual orientation has an early neurodevelopmental basis. Handedness, a behavioral marker of early neurodevelopment, has been associated with sexual orientation in some studies but not in others. The authors conducted a meta-analysis of 20 studies that compared the rates of non-right-handedness in 6,987 homosexual (6,182 men and 805 women) and 16,423 heterosexual (14,808 men and 1,615 women) participants. Homosexual participants had 39% greater odds of being non-right-handed. The corresponding values for homosexual men (20 contrasts) and women (9 contrasts) were 34% and 91%, respectively. The results support the notion that sexual orientation in some men and women has an early neurodevelopmental basis, but the factors responsible for the handedness-sexual orientation association require elucidation. The authors discuss 3 possibilities: cerebral laterality and prenatal exposure to sex hormones, maternal immunological reactions to the fetus, and developmental instability.

Birth order and fluctuating asymmetry: a first look.

We investigated the hypothesis that maternal immunoreactivity to male-specific features of the foetus can increase developmental instability. We predicted that the participants' number of older brothers would be positively related to the fluctuating asymmetry of ten bilateral morphological traits. The participants were 40 adult male psychiatric patients and 31 adult male hospital employees. Consistent with the hypothesis, the participants' number of older brothers--but not number of older sisters, younger brothers or younger sisters--was positively associated with fluctuating asymmetry. The patients had significantly larger fluctuating asymmetry scores and tended to have more older brothers than the employees, but the positive relationship between the number of older brothers and fluctuating asymmetry was observed in both groups.

Psychopathy and developmental instability.

Psychopaths are manipulative, impulsive, and callous individuals with long histories of antisocial behavior. Two models have guided the study of psychopathy. One suggests that psychopathy is a psychopathology, i.e., the outcome of defective or perturbed development. A second suggests that psychopathy is a life-history strategy of social defection and aggression that was reproductively viable in the environment of evolutionary adaptedness (EEA). These two models make different predictions with regard to the presence of signs of perturbations or instability in the development of psychopaths. In Study 1, we obtained data on prenatal, perinatal, and neonatal signs of developmental perturbations from the clinical files of 643 nonpsychopathic and 157 psychopathic male offenders. In Study 2, we measured fluctuating asymmetry (FA, a concurrent sign of past developmental perturbations) in 15 psychopathic male offenders, 25 nonpsychopathic male offenders, and 31 male nonoffenders. Psychopathic offenders scored lower than nonpsychopathic offenders on obstetrical problems and FA; both psychopathic and nonpsychopathic offenders scored higher than nonoffenders on FA. The five offenders from Study 2 meeting the most stringent criteria for psychopathy were similar to nonoffenders with regard to FA and had the lowest asymmetry scores among offenders. These results provide no support for psychopathological models of psychopathy and partial support for life-history strategy models.

The sexual preferences of incest offenders.

Inclusive fitness theory suggests that discriminative solicitude and inbreeding avoidance are important mechanisms regulating parent-children interactions. From an inclusive fitness perspective, sex with one's biological children is paradoxical. The authors hypothesized that incest can occur when these mechanisms are not activated (e.g., if a father is uninvolved in child rearing) or are overwhelmed by another factor, such as pedophilic interest. They predicted that biological fathers, who presumably have been the most involved in the rearing of their victims, would show greater phallometrically measured pedophilic interest than would other incest offenders against children (e.g., grandfathers, uncles, stepfathers). The prediction was not supported. A testable alternative hypothesis to explain biological father incest is presented and the importance of assessing pedophilic interest among incest offenders is discussed.

Perceived crime severity and biological kinship.

Two predictions concerning the perceived severity of crimes can be derived from evolutionary theory. The first, arising from the theory of inclusive fitness, is that crimes in general should be viewed as more serious to the degree that the victim is genetically related to the perpetrator. The second, arising from the deleterious effects of inbreeding depression, is that heterosexual sexual coercion should be perceived as more serious the closer the genetic relationship of victim and perpetrator, particularly when the victim is a female of fertile age. Two hundred and thirty university students estimated the magnitude of the severity of brief crime descriptions in three separate studies. In the first two, the biological kinship of victim and perpetrator was varied, and in the third, the hypothetical genetic relatedness of the subject and the fictitious victim was varied. All three studies found the linear relationships between biological kinship and perceived crime severity predicted by theory.

Why children from the same family are so different from one another : A Darwinian note.

The well-established finding that siblings growing up in the same family turn out to be very different from one another has puzzled psychologists and behavior geneticists alike. In this theoretical note we describe the possible ontogeny and phylogeny of a sibling differentiation mechanism. We suggest that sibling competition for parental investment results in sibling differentiation on a number of characteristics, producing different developmental trajectories within families. Variations in developmental trajectories within families may have had fitness advantages in ancestral environments because(a) sibling competition for extrafamilial resources would be reduced and(b) these variations would be suited to environments containing a variety of niches or to changing environments. Predictions derived from this model and an example of an application to attachment theory are presented.

The discriminative validity of a phallometric test for pedophilic interests among adolescent sex offenders against children.

The authors examined the responses of adolescent sex offenders against children on a phallometric test of pedophilic interests. Participants were 40 adolescent sex offenders against children, 75 young adult sex offenders against children, and 39 young adult comparison participants. The responses of adolescents with female victims resembled those of comparison participants; adolescents with any male victims had larger relative responses to child stimuli than comparison participants. Young adult offenders, regardless of victim sex, had larger relative responses to child stimuli than comparison participants. Using a cut score of 0 (indicating equal or greater arousal to children than to adults), sensitivity was 42% for adolescents with any male victims, and specificity was 92% for the comparison participants. Results suggest phallometric testing can identify pedophilic interests among these adolescent sex offenders.

Pavlovian conditioning of sexual interests in human males.

Pavlovian conditioning of the human male sexual response may be involved in the ontogenetic development of sexual interests and may be responsible for individual differences. We attempted to demonstrate Pavlovian conditioning of sexual interests in a nonclinical sample of adult males. Ten participants were exposed to 11 pairings of a slide of a moderately attractive, partially nude female adult (TARGET) and a highly arousing videotape depiction of heterosexual sexual interactions (US). Ten other participants were exposed to 11 presentations of the TARGET alone. Participants exposed to the TARGET-US contingency showed a 10% relative increase in sexual arousal to the TARGET; participants exposed to the TARGET-ALONE contingency showed an 11% relative decrease in sexual arousal to the TARGET. This group difference is interpreted as resulting from both conditioning and habituation.