In many amazonian tribes, they believe in multiple paternity(man men can be the father). When the women are ovulating(most fertile) would they desire sex with the men with the best genetics( most physically attractive) and when they are not ovulating would they desire sex with less attractive men so these men would think the children are atleast partly there's so they provide resources for the children. And as women are most fertile during ovulation the attractive men are most likely to be the biological farther. And the less attractive men that had sex with the female during her non fertile stage would be cucked.