Cross-cultural patterns and the search for evolved psychological mechanisms

Darwin's principle of evolution by natural selection provides a theoretical basis for functional analyses of behaviour. This approach is complementary to traditional psychology: ideas about what behaviour was designed to do suggest how it might be organized. The cross-cultural record, because it focuses on the broadest characterization of human behaviour, can guide our search for psychological adaptations. Both universals (behavioural traits that are invariantly expressed despite cultural diversity) and conditional universals (behaviours that vary predictably with some environmental parameter) deserve attention from evolutionary psychologists. These behaviours are not themselves adaptations but are markers for underlying psychological traits that have been favoured by selection because they produce adaptive behavioural output. One universal (reciprocity) and one conditional universal (matrilineal investment bias) are used to exemplify how behavioural universals can guide the search for psychological adaptations.

Human marriage systems and sexual dimorphism in stature

Contemporary populations of Homo sapiens are sexually dimorphic on a variety of traits. In terms of stature, men are reliably between 4% and 10% taller than women in well-sampled human populations. Are cross-cultural differences in the magnitude of sexual dimorphism consistent with expectations from sexual selection theory? Prior studies have provided conflicting answers to this question in part because they failed to agree on how the force of sexual selection should or could be operationalized. Here we offer a simple and unbiased method for operationalizing sexual selection and retest two separate predictions from earlier work (Alexander et al., 1979) about its expected impact on stature dimorphism in a sample of 155 societies. Neither prediction matches the observed cross-cultural distribution of dimorphism. However, this is not the consequence of a random distribution of dimorphism across societies. Instead, the data exhibit a robust and unexpected pattern.

Spatial memory and adaptive specialization of the hippocampus

The hippocampus plays an important role in spatial memory and spatial cognition in birds and mammals. Natural selection, sexual selection and artificial selection have resulted in an increase in the size of the hippocampus in a remarkably diverse group of animals that rely on spatial abilities to solve ecologically important problems. Food-storing birds remember the locations of large numbers of scattered caches. Polygynous male voles traverse large home ranges in search of mates. Kangaroo rats both cache food and exhibit a sex difference in home range size. In all of these species, an increase in the size of the hippocampus is associated with superior spatial ability. Artificial selection for homing ability has produced a comparable increase in the size of the hippocampus in homing pigeons, compared with other strains of domestic pigeon. Despite differences among these animals in their histories of selection and the genetic backgrounds on which selection has acted, there is a common relationship between relative hippocampal size and spatial ability.

Trivers-Willard effect in contemporary North American society

The Trivers-Willard hypothesis joins the ideas of R.A. Fisher and A.J. Bateman to model parental investment strategies. Trivers and Willard argue that any overall investment bias favoring either daughters or sons would be maladaptive. Nevertheless, they suggest that, in effectively polygynous species, more complex, conditional sex biases could be adaptive. In particular, they predict that parents in good condition will bias their investment toward sons and that parents in poor condition will bias their investment toward daughters. Among a sample of approximately 900 U.S. mothers we examined several measures of maternal investment including birth weight, interbirth interval and lactational commitment. Maternal condition was assessed by income and by the presence or absence of a coresident adult male. Some measures of investment (five of 14 statistical tests) showed marked and significant sex-by-condition interactions of the type and in the direction predicted by Trivers and Willard; none showed significant effects in the opposite direction. No conscious mediation is required to produce the observed investment patterns.

Evolution of spatial cognition: sex-specific patterns of spatial behavior predict hippocampal size

In a study of two congeneric rodent species, sex differences in hippocampal size were predicted by sex-specific patterns of spatial cognition. Hippocampal size is known to correlate positively with maze performance in laboratory mouse strains and with selective pressure for spatial memory among passerine bird species. In polygamous vole species (Rodentia: Microtus), males range more widely than females in the field and perform better on laboratory measures of spatial ability; both of these differences are absent in monogamous vole species. Ten females and males were taken from natural populations of two vole species, the polygamous meadow vole, M. pennsylvanicus, and the monogamous pine vole, M. pinetorum. Only in the polygamous species do males have larger hippocampi relative to the entire brain than do females. Two-way analysis of variance shows that the ratio of hippocampal volume to brain volume is differently related to sex in these two species. To our knowledge, no previous studies of hippocampal size have linked both evolutionary and psychometric data to hippocampal dimensions. Our controlled comparison suggests that evolution can produce adaptive sex differences in behavior and its neural substrate.

Effects of experience and motivation on symmetrical-maze performance in the prairie vole (Microtus ochrogaster)

Sex differences in spatial skills are sometimes attributed to sex differences in spatial experience. This hypothesis rests on two assumptions: Spatial experience typically differs with sex and spatial experience has lasting effects on spatial cognition. We tested the latter assumption in a controlled experiment with wild-caught prairie voles (Microtus ochrogaster) and their spatially deprived, laboratory-reared, first-generation offspring; we found the assumption to be unjustified. Although major differences in spatial experience had no effect on maze performance, relatively small differences in motivation produced a significant difference in error rates.

Sex differences in spatial ability and activity in two vole species (Microtus ochrogaster and M. pennsylvanicus)

The hypothesis that sex differences in maze learning result from sex differences in activity was tested with wild-caught prairie (Microtus ochrogaster) and meadow (M. pennsylvanicus) voles. For 38 voles error production and activity were simultaneously measured in a series of 7 symmetrical mazes. Repeated-measures analyses of variance (ANOVAS) examined species, sex, maze, and interaction effects for 3 dependent variables: errors, activity, and errors/activity. The pattern of significant effects was very different for the errors and activity ANOVAS, which suggests that differential activity cannot explain differential error rates. In contrast, the pattern of effects was very similar for errors and errors/activity ANOVAS, which suggests that controls for activity do not remove differences in error production. These results fail to support the activity hypothesis.