Parasite-stress theory

Parasite-stress theory, illustrated by researchers Corey Fincher and Randy Thornhill, is a theory of human evolution proposing that parasites and diseases encountered by a species shape the development of species' values and qualities. The differences in how parasites and diseases stress people's development is what leads to differences in their biological mate value and mate preferences, as well as differences across culture. Parasites causing diseases pose potential ecological hazards and, subsequently, selection pressures can alter psychological and social behaviours of humans, as well as have an influence on their immune systems.

Several hypotheses have attempted to explain how parasite load influences female mate choice, as certain traits are thought to be costly and the expression of such traits may be indicative of genetic quality.

According to the Hamilton-Zuk hypothesis, female mate choice is based on the extent to which male secondary sexual characteristics are expressed, as these are thought to be indicative of a heritable resistance to pathogens. A meta-analysis reviewed studies exploring the magnitude of the relationship between expression of secondary sexual characteristics and parasite intensity, as well as level of host immune functioning. Consistent with the hypothesis proposed by Hamilton and Zuk, the meta-analysis revealed that males with the fewest parasites and/or the strongest immune systems typically had the most extravagant secondary sexual characteristics. With regards to parasite-stress theory, these findings would be interpreted as those men who have encountered more parasites – or are naturally less capable of dealing with parasites – are also less desirable mates to women, due to a lower quality genetic quality for the potential offspring.

The Zahavi handicap principle, originally proposed by Zahavi in 1975, suggests that males who possess secondary sexual characteristics which provide a handicap are more attractive to females. These sexual ornaments are sexually selected in order to appear stronger and better adapted, compared to other males in the environment. This is because these characteristics are indicators of good genes and heritable viability, as they are costly to an individual's survival to maintain and produce. Therefore, the stronger the individual is, the more able they are to bear this cost.

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