The aphorism “birds of a feather flock together” describes the fact that people tend to prefer associating with others who are similar to themselves. The phenomenon goes by different names: Sociologists call it homophily, psychologists call it in-group favoritism and political scientists call it affective polarization. It’s observed in a wide range of demographic and social characteristics including sex, race, religion, age, education and political party.

But what about parental status? Do parents prefer other parents? What about child-free people who don’t want to be parents? Do these preferences even matter?

Pronatalism, a set of beliefs and political policies that promotes and favors human reproduction, is common in many countries. Therefore, it’s not surprising that people tend to have more positive attitudes about parents than they do about child-free people.

For example, people generally perceive parents as kinder and more psychologically fulfilled than child-free people. Additionally, people express feelings of admiration toward mothers and feelings of disgust toward child-free women.

However, these are general attitudes and don’t tell us about how people feel about others who have made the same reproductive choices as themselves. That’s why, in a 2022 study of 1,500 Michigan adults, we asked parents how they felt toward other parents and toward child-free people. We also asked child-free people how they felt toward other child-free people and toward parents.

We found that parents strongly favored other parents, but child-free adults didn’t necessarily favor other child-free adults. That is, parents exhibit in-group favoritism, but child-free adults don’t.

A “feeling thermometer” question is one common way to measure how people in one group feel about people in their own group or in other groups. This question asks a person to rate how warmly they feel toward a group on a scale from 0, or very cool, to 100, or very warm.

For example, in 2017 the Pew Research Center asked people how they felt about members of their own religion and members of different religions. White evangelicals reported feeling very warm toward other white evangelicals, with an average warmth score of 81. Likewise, atheists reported feeling very warm toward other atheists, with an average warmth score of 82.

This is evidence of in-group favoritism. At the same time, evangelicals reported feeling very cool toward atheists, with an average warmth score of only 33. Likewise, atheists reported feeling very cool toward evangelicals, with an average warmth score of only 29. This is evidence of what’s called “out-group derogation” – people dislike members of other groups.

We used the same approach to compare parents and child-free adults, and discovered three important patterns.

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