For those who haven't read the article, here's a synopsis: this study used 377,000 high school students from different families that all had multiple children, (i.e) they studied the difference in IQ and personality from an older sibling to a younger sibling from a different family.
After reading this, I feel like they should have done the "within families" method. (Using siblings from the same families.)
From what I could understand, their counter-argument for this was, "such studies often don't measure the personality of each child individually," and that, "They just ask one child -- usually the oldest, 'Are you more conscientious than your siblings?'" I believe they could have done a "within family" method and just created better personality questions for the study.
The researchers also said that the older sibling is older and has a different personality, so a parent would respond to a survey with, "But my oldest kid is more responsible than my youngest kid." What kills me is that this is the very next paragraph:
An ideal within-family study would follow the families over time, collecting IQ and personality data from each child when he or she reached a specific age, the researchers said.
It just seems to me that these researchers weren't really paying attention to all the pros and cons of the two different studies.