Getty Images

Moms and dads offer more mental stimulation to their firstborn, researchers say, and that can have lasting effects.

Amanda MacMillan
February 10, 2017

Sorry, younger siblings: Research shows that firstborn children tend to outperform their brothers and sisters in school, and earn more money as adults. But if it makes you feel better, you can blame Mom and Dad. A new study suggests that the differences between siblings begin in early childhood—and that they have a lot to do with how parents treat their kids.

Although middle and younger children receive the same level of emotional support from parents as firstborns do, they get less support for tasks that develop thinking skills (like reading and mentally stimulating play), according to the authors.

Some of that may be inevitable, as parents with larger broods have less time overall to devote to each child individually. But knowing that the discrepancy exists—and that it may have lasting effects—could help families make informed choices about the time they do spend together, the researchers say.

For their study, they tracked nearly 5,000 children in the United States from before birth through age 14. Every two years they tested the kids on reading and cognitive skills, and gathered information about environmental factors (such as economic conditions and family background).

RELATED: 6 Ways Your Siblings Make You Who You Are

They found that, as early as age 1, firstborn children tended to score higher on IQ tests than their younger siblings. These differences increased slightly until the children entered school; and they remained evident for as long as the kids were followed.

“The fact that the differences in these outcomes start incredibly early was very surprising to us,” first author Jee-Yeon Lehmann, PhD, an economist at Analysis Group, Inc. in Boston, told Health. And the finding may help explain the “birth order effect” identified in other studies, she says, in which firstborn children grow up to make more money and complete more years of education.

When the researchers looked the parents, they found that mothers and fathers devoted the same amount of quality time to all of their children. But, on average, they offered less mental stimulation to younger siblings. “We’re talking about things like how much time you spend reading with your kids, how often you go on cultural outings, if you have books and crafts and musical instruments around,” says Lehmann.

“We found that parents are more likely to spend their time and resources providing these beneficial inputs for their older children than their younger ones,” she adds. And in an analysis that controlled for these behaviors, the cognitive differences between firstborn and later-born children were largely diminished—indicating that they do seem to be directly related.

RELATED: 30 Signs You're in a Toxic Relationship

The researchers also found that women were more likely to engage in riskier behaviors—like smoking and drinking—during pregnancies after their first, and they were less likely to breastfeed later-born children, as well. These specific factors seemed to only have a small impact on children's outcomes, however. 

The study was co-authored by economists at the University of Edinburgh and the University of Sydney, and was published in The Journal of Human Resources. Lehmann calls the findings “kind of an intuitive story.”

“I’m a mother of two kids myself, and I certainly had much more time to spend with my first child in terms of reading and those types of activities,” she says. “Now that life is really busy, it’s harder to invest the time and resources in the areas that are, in some ways, the nonessential parts of raising your kids.”

But parents with more than one child shouldn’t freak out: Lehmann stresses that the differences noted between siblings in the study were slight. “Children coming from good families tend to be high achieving across the board,” she says. “It’s not like the older siblings are going to college and the younger ones are dropping out of middle school.”

For more news you need to know, sign up for our Healthy Living newsletter

And while limits on time and resources may dictate how parents raise their kids, Lehmann says there’s room for parents to be cognizant of the importance of early development.

“I’ve been making slightly different choices based on our findings, and recognizing that maybe I am spending less time doing these explicit educational activities with my second child,” she says. “The paper’s findings have given me more motivation to find those moments whenever I can.”