Poor only children. So deprived of sibling love and companionship. So maladjusted compared to their peers.

Or so conventional thinking goes.

The authors of “NurtureShock” take a hard look at studies of only children as well as research on siblings. What they find runs counter to what everyone seems to assume about growing up with brothers and sisters:

… [T]he theory that being an only child deprives a child of social skills makes so much logical sense. By growing up with siblings, a child has thousands upon thousands of interactions to learn how to get along. According to this theory, children with siblings should be massively more skilled at getting along than children with no siblings. Yet they aren’t.

Studies have shown siblings between ages 3 and 7 fighting an average of 3.5 times an hour, or about 10 minutes of every 60. Hardly surprising. But seven out of eight of these clashes don’t end with compromise or any other outcome that satisfies both children—“the siblings withdraw, usually after the older child has bullied or intimidated the younger.”

Why aren’t children with siblings learning to get along? Simply put, kids know their siblings are in it for the long haul, research suggests. While they may learn that they can only push a peer so far without losing their friendship, there’s no comparable threat with siblings. In fact, a child can experiment by pushing a sibling to their limits without fear of social rebuke.

And just what are our kids fighting about? Well, not about us. It’s easy to assume there’s an underlying struggle for parental affection at work, but the simplest explanation is the truest, in this case: Kids fight about toys and other possessions. In fact, they fight about things the vast majority of the time. And they fight about fighting itself. In a study of 108 sibling pairs, fighting over parental affection barely registered—it was last among reasons, with 9 percent of children claiming it was behind their fights.

Some parents try to minimize conflict between siblings by spacing out their children. Some have children closer together, thinking their kids will stand a greater chance of being friends. Some try to space them farther apart, reasoning that there will be less direct competition, and thus fewer conflicts. But research on whether sibling spacing has any effect on relationship quality is mixed. The same holds for the siblings’ genders.

It’s also natural to cut ourselves some slack when our children are young. Surely the kids will grow out of this, we think. It’s just a matter of time. 

Again, research doesn’t back this up. A long-term study of sibling relationships found that, good or bad, they are “remarkably stable.” Dr. Laurie Kramer of the University of Illinois scored the same siblings’ interactions several times over 13 years. Among all 30 families studied,

Unless there had been some major life event in the family—an illness, a death, a divorce—the character of the relationship didn’t change until the eldest moved out of the house. For the most part, the tone established when they were very young, be it controlling and bossy or sweet and considerate, tended to stay that way.

It turns out the biggest predictor for how children will treat siblings is how they act with their best friends. If a child has a decent level of respect and give and take with that friend, research has found, chances are better that he will be able to build a similar relationship with a sibling. According to conventional thinking, siblings build their social skills by practicing with each other and applying that knowledge elsewhere, but it’s the actually the opposite that’s true—kids use what they learn with their friends on their siblings.

Aside from encouraging deeper relationships with friends, parents can also promote fantasy play as a way to develop social skills that enhance sibling relationships. Fantasy play takes commitment on all counts, the authors say. “When one kid just announced the beginning of a ninja battle, but the other wants to be a cowboy, they have to figure out how to still ride off into the sunset together.”

Does anything surprise you about the research on sibling relationships? What kind of relationship did you have with your siblings growing up?