First-born children stand a better chance of academic success — and making more money as adults — than their younger siblings, an economics professor has found.

In findings collected since 2004 and posted by the National Bureau of Economic Research, Sandra Black and fellow researchers found parents are in some ways more invested in their first born than kids born later down the line.

"First-born children do better on education, on earnings," Black told Marketplace. "And what we see also is that there is a declining pattern by birth order. So, second-born does a little bit worse than the first-born, the third-born does a little bit worse. And it kind of goes down the birth order till about the fourth-born, and then it flattens out."

"We find some evidence that suggests that parents invest differently with earlier-born children," she added. "We see that parents spend more time helping them with homework. The problem is it's hard to distinguish whether that's the parents treating them differently, or the kids are different and the parents are just responding to it."

The research also found the birth-order effects are worse for women, with later-born females earning less, less likely to work full-time, and more likely to have their first child in their teens. Later-born men have lower full-time earnings, but they are not less likely to work full-time, the researchers found.

The econ professor warns the results will not apply to every family, telling Marketplace, "Every time I talk about this, people kind of write about their stories, and so I want to caution that that this is talking on average. Even if your family is not specifically like this, it still can be true on average."

A 2015 review of studies on birth order, found birth order has no effect on IQ or personality.

"You're not going to be able to sit two people [of different birth order] down next to each other and see the differences between them. It's not noticeable by anybody," study co-author Brent Roberts told Business Insider at the time.

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