Parents are toughest on their eldest child, according to new research.

Parents pay more attention to their first-born children's homework and TV habits, resulting in the eldest child performing better in school. First-borns are also subject to their parent's extra nagging.

Researchers V. Joseph Hotz, a professor of economics at Duke University, and Juan Pantano, an assistant professor of economics from Washington University in St. Louis, looked at data from the National Longitudinal Survey of Youth, 1979, which includes details from women about all of their children.

The mothers involved in the study provided info that seems consistent with what's known about birth order and academics; the oldest child performs best, the next oldest, a little worse, and so on. The information from the longitudinal study is self-reported, meaning the moms report how well their children do rather than provide test scores or report cards.

"It's very, very interesting," Toni Falbo, professor of educational psychology at the University of Texas, Austin, who has no affiliation with the study, told "Today". "It is not inconsistent with what we know in psychology."

Many studies have shown that first-born children have higher IQs, do better in school and are perceived as more successful by their parents. But this study also applied an economic look at what psychologists have tackled for years, by linking parenting style to academic success.

"Economists view parents as using rewards and penalties to manipulate their children's performance in school and these rewards/penalties provide differential incentives across birth order," Hotz wrote in an email.

Most studies about birth order and intelligence show that first-born children are smarter than the subsequent ones, but don't explain why. Theorists have come up with some ideas: the dilution hypothesis, for example, says the second child has less time with and attention from the parents, and the third has even less.

While the Hotz and Pantano admit that the dilution hypothesis might have an impact on intelligence based on birth order, they believe the strongest contributor to older children doing better in school is how much the parents watch them.

First-born children likely live in a tightly regulated house, with specific rules and scrutiny about everything from TV watching to homework. And, moms said they were more likely to closely watch an earlier-born child who brought home a bad report card.

"This study stands out because it has the data that others don't have," said Frank Sulloway, an adjunct professor of psychology at the University of California, Berkley, who did not participate in the study. "What this study actually has is a lot of data about what the mothers and fathers are doing [with their children]."