The website Study Finds produces summaries of science research, publishing multiple summaries per day. This example concerns a study showing that how many kids people have had (in their younger years) predicted their later-life cognitive function. Somewhat confusingly, the print headline reads, "having more kids linked to weaker brain function," but the accompanying video segment includes the causal claim, "Kids can weaken your brain".
Here's the opening segment from the journalist:
Everything in moderation — even kids? Researchers from Columbia University and Université Paris-Dauphine report having more than two kids may have a negative impact on late-life cognition. The study shows that older parents with just two children appeared sharper cognitively than those with three.
a) There are two main variables in this statement so far. What are they? Do you think these variables were manipulated or measured?
b) Identify the verbs in the bolded statements above, and explain which ones are association claim verbs and which are causal claim verbs.
c) On the face of it, the study found a simple correlation between having more than two kids (variable A) and cognitive function (variable B). You know that correlation is not causation, of course. What are some possibilities that explain this relationship? Consider temporal precedence first: Come up with a causal story in which A leads to B, as well as an alternative story in which B leads to A. Then come up with a C variable (a third variable, or internal validity problem) that might plausibly be related to both A and B.
Now that you've assessed the basics of this bivariate correlational study, here's more of what the journalist said about the study:
...study authors analyzed data provided by the Survey of Health, Aging and Retirement in Europe (SHARE). Featuring representative samples of the older populations across 20 European countries and Israel, SHARE was an ideal match for this project. Each individual was at least 65 years old and had at least two biological children.
Via a series of complex econometric methods, the research team was able to successfully disentangle various causal relationships from simple associations. In simpler terms, the analysis found that having three or more kids (as opposed to just two) is related to worse late-life cognition. This held up among both moms and dads.
[...]
“The negative effect of having three or more children on cognitive functioning is not negligible, it is equivalent to 6.2 years of aging,” Dr. Bonsang notes.
The journalist referred to "complex econometric methods" because the study itself used a creative strategy to try to test the relationship of having more than two kids on cognitive functioning, while ruling out potential alternative explanations. The researchers started with the assumption that parents who choose to have more than one kid often wish to have at least one boy and one girl. Indeed, in their large European data set, they found evidence that when people start out with two kids of the same sex (such as two boys or two girls), they are more likely to go for a third child, compared to people whose first two kids were a boy and a girl. Then, when the researchers statistical controlled for this variable (the sex composition of the first two kids), they observed a strong decline in cognitive function in later years.
The upshot is that the researchers tried to take advantage of a natural experiment that goes something like this: When people's first two kids are the same sex, they're more likely to have a third child, which is likely to be followed by cognitive decline in later years. In contrast, when people's first two kids are different sexes, they're less likely to have a third child, which is followed by less cognitive decline. Because sex of one's first two children is more or less randomly assigned, it serves as an experimental variable.
Importantly, they found the effect of children on cognitive decline only in the Northern European countries (not other regions such as Western, Eastern, or Southern).
Here are some brief questions for you.
Which of the four big validities (internal, external, construct, or statistical) is each of these statements addressing? Each quote comes from the journalist's coverage.
d) "...study authors analyzed data provided by the Survey of Health, Aging and Retirement in Europe (SHARE). Featuring representative samples of the older populations across 20 European countries and Israel" (Which validity is this?)
e) “'The negative effect of having three or more children on cognitive functioning is not negligible, it is equivalent to 6.2 years of aging,' Dr. Bonsang notes." (Which validity is this?)
f) Which two validities are left? Ask a question that would provide information about the remaining two validities.
g) Is it a problem that this effect was found only in the Northern European sample, and not the other three samples? Why or why not?
