Specifically, they wanted to compare their major histocompatibility complexes, which are immune-system genes.
The researchers found that the more similar a couple's MHCs were, the less attracted the participants were to their partners and the more likely they were to have had sex outside the relationship.
Sleep-deprived people were rated as less healthy and less attractive.
Three years later, the researchers went into more detail, and other participants rated the people in the photos based on different criteria.
Participants rated people as least attractive when they were described as evil and mean.
The "power pose" is a controversial topic in the scientific community.
Some of those photos were accompanied by the Chinese words for "decent" and honest." Some were accompanied by the Chinese words for "evil" and "mean." Others weren't accompanied by any information.
Results showed that men rated women most attractive when they looked happy and least attractive when they displayed pride.
Women, on the other hand, rated men most attractive when they displayed pride and least attractive when they looked happy.
And men pictured in contractive postures seemed to be at a special disadvantage. A 2013 study by researchers in Finland, South Africa, the UK, Latvia, and Estonia found that Latvian women with high levels of the stress hormone cortisol were perceived by heterosexual men in Latvia as less attractive.
Even though the men didn't know the women's cortisol levels — only the researchers did — it seemed to affect the women's perceived attractiveness.