Evolutionary theories suggest that humans prefer sexual dimorphism in faces because masculinity in men & femininity in women may be an indicator of immune function during development; Australian data seems to confirm this

Immune function during early adolescence positively predicts adult facial sexual dimorphism in both men and women. Yong Zhi Foo et al. Evolution and Human Behavior, February 17 2020. https://doi.org/10.1016/j.evolhumbehav.2020.02.002

Abstract: Evolutionary theories suggest that humans prefer sexual dimorphism in faces because masculinity in men and femininity in women may be an indicator of immune function during development. In particular, the immunocompetence handicap hypothesis proposes that sexual dimorphism indicates good immune function during development because the sex hormones, particularly testosterone in men, required for the development of sexually dimorphic facial features also taxes the immune system. Therefore, only healthy males can afford the high level of testosterone for the development of sexually dimorphic traits without compromising their survival. Researchers have suggested that a similar mechanism via the effects of oestrogen might also explain male preferences for female femininity. Despite the prominence of the immunocompetence handicap hypothesis, no studies have tested whether immune function during development predicts adult facial sexual dimorphism. Here, using data from a longitudinal public health dataset, the Western Australian Pregnancy Cohort (Raine) Study (Generation 2), we show that some aspects of immune function during early adolescence (14 years) positively predict sexually dimorphic 3D face shape in both men and women. Our results support a fundamental assumption that facial sexual dimorphism is an indicator of immune function during the development of facial sexual dimorphism.