Parasite-stress theory
[1] 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.
According to the Hamilton–Zuk hypothesis,[3] 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[4] 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.
Examples of such traits include the peacock's tail, very bright in nature and hence attracting more attention from predators as well as requiring more energy to maintain.
This hypothesis takes Zahavi's principle further in suggesting that testosterone is responsible for the production of male secondary sexual traits while also suppressing the immune system.
It therefore proposes that these traits are honest signals of mate quality because only males with 'good genes' should be able to fully express them without being vulnerable to parasite attack.
[7] Males will, therefore, demonstrate their high genetic quality by developing more attractive honest signals in substitute for their immune system's strength.
The hypothesis states that parasites indirectly influence literally all of the main processes that otherwise underlie the domestication syndrome (abnormalities in the functioning of the neuro-neuroendocrine system, a developmental disruption of neural crest cell input to the affected phenotypic traits, etc.).
[10][11] Developmental instability is the inability of an organism to produce its optimal phenotype, due to genetic limitations and environmental stresses (such as parasite load).
[12] A literature review summarising more than 100 different studies in the field found that, among other variables, immunocompetence (the ability of an organism to produce a normal immune response to an antigen) had a significant relationship with fluctuating asymmetry.
Hence, ensuring that those in the in-group are not affected by a novel disease will subsequently result in a reduced risk of encountering a novel parasite from an exposed person an individual remains in close proximity with.
[23] Several scientists have criticized the theory that pathogen stress can explain differences in collectivism versus individualism, suggesting that the observed correlations were spurious.
The more-efficient government institutions inherited from colonial times in low mortality countries can explain the observed differences in cultural values.
