A researcher tests all the students in one college and finds that the first-borns have higher GPAs than later-born children, on the average. One possible explanation is that the results reflect the influence of birth order. The other explanation is that the results depend on differences between

Question

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The birth order effect could be confounded by the fact that one-child families may be over-represented in the group of later-born children, which could result in lower GPAs for later-born children due to lack of siblings. Therefore, the observed differences in GPAs may not be solely due to birth order, but also to the family size.

A Researcher Tests All the Students in One College and Finds