Linoleic and docosahexaenoic acids in human milk have opposite relationships with cognitive test performance in a sample of 28 countries.

Polyunsaturated fatty acids play critical roles in brain development and function, and their levels in human breast milk closely reflect the long-term diet. The fatty acid contents of human milk samples from 28 countries were used to predict averaged 2009 and 2012 test scores in mathematics, reading, and science from the Program for International Student Assessment. All test scores were positively related to milk docosahexaenoic acid (r=0.48 to 0.55), and negatively related to linoleic acid (r=-0.28 to -0.56). Together, these two human milk fatty acids explained 46% to 48% of the variance in scores, with no improvement in predictive power when socioeconomic variables were added to the regression. The (log) ratio of linoleic to arachidonic acid was negatively related to scores (r=-0.45 to -0.48). Statistical effects were similar for the two sexes. In a separate US sample, estimated dietary linoleic was negatively related to the levels of all long-chain n-3 and n-6 plasma fatty acids. High levels of dietary linoleic may impair cognition by decreasing both docosahexaenoic and arachidonic acids in the brain.

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.

Matrilateral biases in the investment of aunts and uncles : A consequence and measure of paternity uncertainty.

In a study of the kin investment of aunts and uncles we show that the laterality effect expected as a result of paternity uncertainty is statistically reliable but somewhat smaller than the sex effect. Matrilateral aunts invest significantly more than patrilateral aunts, and the same is true for uncles. Regardless of laterality, however, aunts invest significantly more than uncles. Multivariate controls show that the matrilateral bias is fully independent of any age or distance confounds that might result from sex differences in age at marriage or dispersal. We discuss our results in relation to recent findings on the kin investment of grandparents (Euler and Weitzel 1996). In addition, we propose a simple method for estimating the level of paternity uncertainty from kin investment data; application of this method to our data on aunts and uncles suggests that between 13% and 20% of children are not the offspring of their putative father. Our parallel analyses of Euler and Weitzel's (1996) data on grandparental investment suggest a similar estimate, that paternity uncertainty lies between 9% and 17%.

Effects of gender and sexual orientation on evolutionarily relevant aspects of human mating psychology.

Sexual selection theory provides a powerful model for the analysis of psychological sex differences. This research examined (a) tests of several sex differences in mating psychology predicted from sexual selection theory, (b) broad developmental hypotheses about sex differences in mating psychology--through the relationship of mating psychology to sexual orientation, and (c) the structure of within-sex differences in mating psychology. Scales measuring aspects of mating psychology were administered to heterosexual and homosexual Ss of both sexes. The structure of scale intercorrelations was similar across groups. All scales yielded sex differences consistent with sexual selection theory. Homosexual Ss generally obtained scores similar to those of same-sex heterosexual Ss, though several scales were significantly related to sexual orientation. Findings constrain hypotheses concerning the origins of sex differences.

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.