Objectives Sexually dimorphic characteristics can signal high genetic fitness and parasite resistance in intersexual selection; and individuals choose and prefer mates who appear resistant to parasites. Furthermore, sexually dimorphic traits can indicate intrasexual competitiveness. Beards are a sexually dimorphic trait, and in the current study we investigate the relationship between pathogen prevalence, income inequality and beardedness in men. Methods Using publicly available data from The World’s Muslims’ dataset (PEW Research Center), encompassing more than 14,000 individuals from 25 countries, we test the hypotheses that men wear beards where the pathogen stress is higher and male-male competition is more prevalent. Results Our results show that beardedness in men is predicted by pathogen prevalence as well as GINI coefficient. Conclusion Overall, the results of this study provide support for the Hamilton-Zuk parasite-mediated sexual selection hypothesis, indicating that men wear beards in environments where the parasitic stress is higher (in order to signal their vigor, health, and ability to resist parasites in such environments), and where there is more prevalence of male-male competition (indicated by higher wealth inequality).

中文翻译：

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寄生虫患病率和收入不平等积极预测25个国家/地区的胡须

目的性两性特征可以在两性选择中反映出高遗传适应性和寄生虫抗性。个人选择并偏爱对寄生虫有抵抗力的伴侣。此外，性二形性特征可以表明性竞争能力。胡须是一种两性性状，在目前的研究中，我们调查了男性病原体患病率，收入不平等和胡须之间的关系。方法使用来自世界穆斯林数据库（PEW研究中心）的公开数据，该数据包括来自25个国家的14,000多名个人，我们检验了以下假设：男人在病原体压力较高且男性与男性竞争更为普遍的情况下留着胡须。结果我们的结果表明，男性的胡子是由病原体流行率和GINI系数预测的。