It turns out a nation of annoyed older siblings might not be wrong; their younger sisters and brothers really might be ruining their lives. "We find that families face a substantial quantity-quality trade-off," researchers write in a newly published study. "Increases in family size decrease parental investment, decrease childhood cognitive abilities, and increase behavioral problems." The Washington Post reports researchers looked at 26 years of data to come to that conclusion. They found that parental investment—specifically the amount of time parents spend with each child—dropped by 3 percentage points for every additional child. Cognitive scores fell 2.8 percentage points. And children received an average of 0.13 years less education for each sibling they had.
The effects of having siblings weren't temporary, either, the Post reports. Children with more siblings had worse lives in the long run, characterized by less money and more run-ins with the law. "Not only do we fail to find evidence that the impact is transitory, effects appear to substantially worsen over the longer run," Business Insider quotes the researchers as saying. However, the negative effects of siblings could be slightly counteracted by having smarter mothers, as measured by the Armed Forces Qualification Test. Researchers theorize that could be because more intelligent mothers tend to have better jobs with increased salaries and more time off to spend with children. (Firstborns tend to have higher IQs, too.)