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Oh, that explains it

March 17, 2009

A new study by Timothy Salthouse adds to the body of work suggesting that raw cognitive performance begins to decline in early adulthood.

News reports are presenting the basic age pattern as a new finding. It’s not, or at least it’s not new in the way it’s being portrayed. The idea that fluid intelligence peaks in the 20s and then declines has been around for a while. I remember learning it as an undergrad. I teach it in my Intro classes.

So why is a new study being published? Because the research, reported in Neurobiology of Aging, tries to tease apart some thorny methodological problems in estimating how mental abilities change with age.

If you simply compare different people of different ages (a cross-sectional design), you don’t know if the differences are because of what happens to people as they get older, or instead because of cohort effects (i.e., generational differences). In other words, maybe members of more recent generations do better at these tasks by virtue of better schooling, better early nutrition, or something like that. In that case, apparent differences between old people and young people might have nothing to do with the process of getting older per se.

To avoid cohort effects, you could follow the same people over time (a longitudinal design). However, if you do that you have to worry about something else — practice effects. The broad underlying ability may be declining, but people might be getting “test-smart” if you give them the same (or similar) tests again and again, which would mask any true underlying decline.

As a result of different findings obtained with different methods, there was a majority view among researchers that fluid performance starts to decline in early adulthood, but also a significant minority view that that declines happen later.

What Salthouse did was to look at cross-sectional and longitudinal data side-by-side in a way that allowed him to estimate the age trajectory after accounting for both kinds of biases. In principle, this should yield more precise estimates than previous studies about the particular shape of the trend. Based on the combined data, Salthouse concluded that the early-adulthood peak was more consistent with the evidence.

It’s understandable, but unfortunate, that the media coverage isn’t going into this level of nuance. Science is incremental, and this study is a significant contribution (though by no means the last word). But news stories often have a set narrative – the lone scientist having a “eureka!” moment with a shattering breakthrough that “proves” his theory. Science doesn’t work that way, but that’s the way it’s usually covered.

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