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If there’s one thing that’s sure to prompt unchecked reactions, it’s violating social scripts. Like, say, the fearsome American expectation of producing progeny. Not only is it atypical, according to a new paper in Sex Roles, being child-free is wrong — to the point of inspiring moral outrage.

That’s quite the finding, given how common child-free adulthood has become in the developed world and domestically. The American birth rate continues to go down — it’s only really getting propped up by migrant moms — and Pew estimates that one in five U.S. women will forgo motherhood altogether.

Author Leslie Ashburn-Nardo, whose school, Indiana University-Purdue University Indianapolis, has the greatest abbreviation in all of academia (IUPUI), observed in her paper that a bias against child-free people had long been documented in the literature, but no one had ferreted out where the distaste comes from. So she did what psychologists do best: fooled some undergraduates.

About 200 Intro to Psych students thought they were participating in a study about intuition. As part of that experiment, they read one of four descriptions of folks who graduated from their same school and went on to live adult lives. The only differences were gender and whether or not the person in question had zero or two kids.

Here’s the description:

James (Jennifer) lives in Columbus, Ohio. He (She) graduated from [name of university] with a degree in biology in December 2002. After graduating, he (she) worked as a pharmaceutical sales rep to pay back some student loans, but after a couple of years he (she) decided that career wasn’t a good fit and investigated other options. In summer 2003, James (Jennifer) married his (her) college girlfriend (boyfriend). They decided to have no (two) children and when surveyed in 2005 they had stuck with this decision.

They then rated how likely James or Jennifer was to be a good parent, how satisfied they and their partner were with marriage, how satisfied they both were about their decision about having kids, how likely they were to get divorced, and how satisfied they were with life overall. They also rated how annoyed, disgusted, or disapproving they felt about the protagonist.

The results: Couples without kids were seen as less psychologically fulfilled, and inspired greater moral outrage. Gender didn’t make a difference.

The reasons for the outrage are quite thoroughly acculturated, she explains. Children learn to identify with the nurturing behavior of their parents, a 1961 study found, and so violating that gets into some deep, knee-jerky psychological territory. Similarly, people do everything because their friends do it, and so seeing your friends popping out humans makes it look like the thing to do. Maybe that’s why, for decades, not having kids has meant regularly fending off questions of “Why don’t you have kids?” Maybe because, according to (some) research, marriage satisfaction goes down with each child. Or, as one respondent told an ethnographer last year: “It’s a rational response to what it means to have a kid and what impact [being a parent] has on the rest of your life.”