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of the pathogen (as is often true of malaria) and the total density of the pathogen is regulated, then wiping out susceptible strains with antibiotics can greatly increase the frequency of formerly rare resistant strains. This raises the possibility that the medical community is ignoring an important human social dilemma: that the best treatment for a patient may not be the best outcome for society as a whole.

Some human disorders can spring not from a failure of adaptation per se, but from disagreement and conflict over what is the correct adaptation. This is particularly so in the realm of human interpersonal relations, starting with fundamental conflicts between parent and offspring. Haig (1993) has argued that such conflicts can lead to pathologies in pregnancy when there is an upset in the precarious resolution of embryo-maternal conflict. Taking a radical step further, he has pointed out that the optimal strategy of an embryo’s gene differs according to whether it came from the dam or the sire, with maternal loci being less selected to take resources from the mother. Remarkably, imprinted genes appear to behave in accord with this theory. In Chapter 12, David Haig extends this thinking in several directions. He notes that most of our kin belong to categories that have asymmetrical relatedness to our maternal and paternal genes, so that most of our psychological adaptations for dealing with kin, and perhaps pathologies, may reflect these kinds of conflicts. In particular, he shows how this perspective may illuminate unsolved problems surrounding the evolution of adolescence and the timing of sexual maturation in humans.

In Chapter 13, Steven Frank and Bernard Crespi extend and generalize the same theme: that conflict can lead to pathologies when opposing interests that are precariously balanced become unbalanced. These authors suggest that the conflict between maternal and paternal genes in offspring, through its demonstrated effects on the regulation and pathologies of growth, may be responsible for some cancers. They then discuss the exciting idea that this same balance is partly responsible for a wide spectrum of psychiatric disorders, such as autism that may result from an overexpression of paternal interests in offspring selfishness. Similarly, other disorders such as schizophrenia might result from an overexpression of genes underlying the maternal goal of greater social integration. Finally, the authors present a novel theory of conflict between autosomal and X chromosomes. The latter spend two-thirds of their time in females and therefore should be selected to give greater weight to female than to male adaptation. Autosomes should give equal weight. It will be fascinating to see if empirical tests support the authors’ prediction that such conflict will underlie pathologies of expression along the male-female axis.

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