Females of many species seek out mates with strong immune systems. Now it appears that female humans do the same thing.

According to the authors of this article a strong immune system plays a larger role in attracting women than muscles or great bodies.

They tested the immune response of 74 young Latvian men to a hepatitis vaccine and measured levels of testosterone and the stress hormone cortisol.

Young Latvian women were asked to rate photos of the men according to attractiveness.

It appears that male facial characteristics signal the strength of the immune system via testosterone.

Men with stronger immune systems had higher testosterone levels. Men with higher testosterone levels were rated as more attractive.

Higher levels of cortisol were associated with lower testosterone levels and lower attractiveness ratings.

This suggests that cortisol could inhibit immune response.