More siblings means less chance of divorce as adult

And the more siblings, the better: Each additional sibling a person has (up to about seven) reduces the likelihood of divorce by 2 percent.

The practical difference between having no siblings and having one or two isn't that much in terms of divorce, said Doug Downey, co-author of the study and professor of sociology at The Ohio State University.

"But when you compare children from large families to those with only one child, there is a meaningful gap in the probability of divorce," he said.

One of the biggest surprises of the study was that it wasn't the difference between being an only child and having siblings that was significant.

"We expected that if you had any siblings at all, that would give you the experience with personal relationships that would help you in marriage," said Donna Bobbitt-Zeher, co-author of the study and assistant professor of sociology at Ohio State's Marion campus.

"But we found that the real story appears to be how family dynamics change incrementally with the addition of each sibling. More siblings means more experience dealing with others, and that seems to provide additional help in dealing with a marriage relationship as an adult."

Downey and Bobbitt-Zeher conducted the study with Joseph Merry, a graduate student at Ohio State. They will present their results Aug. 13 in New York City at the 108th annual meeting of the American Sociological Association.

The study used data from the General Social Survey, which involved interviews with about 57,000 adults from across the United States at 28 points between 1972 and 2012.

The results showed that each additional sibling up to about seven provided additional protection from divorce, Downey said. More siblings than that didn't provide additional protection, although they did not hurt, either.

The good effect of siblings was seen among Americans of all generations studied.

"Siblings help protect against divorce among adults now just as much as they did 50 years ago," Bobbitt-Zeher said.

The researchers took into account a wide variety of other factors that may have affected the results.

"One argument might be that it isn't siblings that matter, but some other difference between large families and small families," Downey said. "It could have been that small families are more likely to have a single parent, or have some other issue that may hurt children in their future marriage relationship."

So the researchers analyzed a variety of variables of both the respondents they surveyed and their parents that could have played a role in future divorces, including education, socioeconomic status, family structure, race, age at marriage, whether the respondents had children, gender role attitudes and religious affiliation, among others.

"When we added in all of these controls, nothing took away the relationship we saw between siblings and later divorce," Bobbitt-Zeher said. "None of these other factors explained it away."

While the study itself can't explain the protective effect of having siblings, Downey said there are good reasons for the findings.

"Growing up in a family with siblings, you develop a set of skills for negotiating both negative and positive interactions. You have to consider other people's points of view, learn how to talk through problems. The more siblings you have, the more opportunities you have to practice those skills," he said.

"That can be a good foundation for adult relationships, including marriage."

Both Bobbitt-Zeher and Downey have done previous research on the influence of growing up with or without siblings.

In 2004, Downey led a study that found kindergarten teachers rated students with siblings as having better social skills than only children. In a study published this year, Downey and Bobbitt-Zeher found that teenagers without siblings didn't seem to have a disadvantage when it came to social skills.

Most other studies of the effects of siblings also look at outcomes in school-aged children, and most show the positive results of smaller families, such as better grades among those with no or fewer siblings.

Downey said this new study is an attempt to both examine the effect of siblings later in life and to see how it impacts more major life events.

"Evaluations of social skills and grades aren't trivial, but divorce is a more concrete, consequential event in a person's life. This is the first study to look at how siblings affect such a consequential event in adulthood," he said.

The results point to one troubling consequence of lower fertility and smaller family sizes in the United States and elsewhere, Downey said.

While much of the work about this demographic shift to smaller families shows the positive side of having fewer children, these findings show there are some negatives to consider as well.

At the same time, the researchers say these results shouldn't lead parents of only children to worry too much.

"There are so many factors that are related to divorce, and the number of siblings you have is just one of them," Bobbitt-Zeher said.

"There is a relationship between the number of siblings and divorce, but it is not something that is going to doom your marriage if you don't have a brother or sister."

The study was supported in part by a grant from the Eunice Kennedy Shriver National Institute of Child Health and Human Development to Ohio State's Institute for Population Research.