Paternity uncertainty is a significant adaptive problem for mammalian males who make extensive parental investment in their offspring (as men do). How can men ever be sure that the children that their wives have given birth to are genetically theirs? Men who are cuckolded, and invest in the genetic offspring of another man, lose both the resources they invest in the children and an opportunity to pass on their genes to their own genetic offspring.

Nature may or may not help the father convince himself of his paternity by making the baby (kind of) resemble the father rather than the mother. However, people (especially maternal kin) appear to help, by telling the father that the baby resembles him, regardless of whether it does or not. After all, the maternal kin, unlike the paternal kin, have no interest in finding out the truth. There is no such thing as maternity uncertainty. — psychologytoday.com