Maturing Intelligence

For decades, many psychologists and neuroscientists have argued that humans have a so-called “cognitive peak.” That is, that a person’s fluid intelligence, or the ability to analyze information and solve problems in novel situations, reaches its apex during early adulthood. But new research done at Massachusetts Institute of Technology and Massachusetts General Hospital paints a different picture, suggesting that different aspects of intelligence reach their respective pinnacles at various points over the lifespan—often, many decades later than previously imagined. Perhaps even more surprisingly, these researchers demonstrated such effects using online intelligence tests.

Historical artifact

More than 70 years ago, psychologist Raymond Cattell and his student John Horn proposed that intelligence was not one thing but rather a collection of various abilities working in concert. Their idea, called the Cattell-Horn theory, proposed that these different abilities could be broadly placed in two categories: fluid intelligence and crystallized intelligence. Fluid abilities include those that allow a person to solve new problems as well as encode short-term memories. Crystallized abilities, on the other hand, are born from knowledge and general information. Cattell and Horn thought that both types of intelligence increased throughout childhood and adolescence. However, while crystallized intelligence would continue to make gains through adulthood, fluid intelligence would peak in the early 20s and then start to decline between ages 30 and 40. This has been the prevailing theory of intelligence since.

“There was just a basic idea that cognition couldn’t get any better after a certain age. Adulthood, when it comes to intelligence, is really boring in a sense,” says Laura Germine, a post-doctoral fellow in psychiatric and neurodevelopmental genetics at Massachusetts General Hospital. “The only thing happening is aging—and that’s just cognitive skills starting to decline.”

Early neuroscientific work in brain development seemed to support the Cattell-Horn theory. “The brain is mostly mature by 18—and then there’s a little more maturation of the frontal lobes into your 20s. But once that happened, then, it was thought, you were done,” says Joshua Hartshorne, a post-doctoral fellow at the Massachusetts Institute of Technology. “So it fit with this idea that nothing was changing until older age.”

Of course, newer investigations are discovering that the brain is much more plastic than once thought. When both Germine and Hartshorne independently unearthed data about fluid abilities peaking later in life, they wondered if there may be more to cognitive peaks than the psychological community had traditionally believed.

Using the web

A few years earlier, Germine and colleagues had posted online a set of facial recognition tests as part of her work in facial blindness and developmental prosopagnosia, or the inability to recognize faces. “We did it as sort of a public service. These were just some tests online that would allow people to assess their face recognition skills,” she says. “But after a while, we were getting lots and lots of people to the website and we realized that we could use the data for science.”

Once Germine and colleagues analyzed the data, they found something odd. The psychological literature held that face recognition ability matures in the late teens, but this data set suggested that it improved into the early 30s. “We thought, ‘Okay, this can’t be right,’” says Germine. “Because the basic idea is that cognition just doesn’t look this way.”

But the more the group looked at the data, the more the trend stood out: Face recognition ability peaked late. Germine and colleagues published these results in February 2011 issue of Cognition. Around that same time, Hartshorne was using the Web to collect his own data on visual working memory, and was also seeing a late peak. “[Germine] and I started talking about it and realized that when you look through the literature, there was enough data to suggest that fluid intelligence peaked early but not really enough to say when or even say do all types of fluid intelligence peak around the same time,” says Hartshorne. “So we wanted to look at those questions in a systematic way.”

To do so, the duo used websites to run a variety of intelligence tests, investigating fluid skills like short-term memory, processing speed, and evaluating emotional states as well as a vocabulary test and other crystallized intelligence measures.

“In many laboratory studies, you see a very heterogeneous population: 18-30 years of age, college educated. You really are getting only the young adults,” says Hartshorne. “But if you run your experiments online, you can let anyone who wants to participate play. And you get a wide range of ages and backgrounds.”

The group collected data from nearly 50,000 participants who logged in to play, and found that different cognitive skills peaked at different times over the lifespan. For example, while short term memory appears to peak at 25 and start to decline at 35, emotional perception peaks nearly two decades later, between 40 and 50. Almost every independent cognitive ability tested appears to have its own age trajectory. The results were reported earlier this year in Psychological Science.

“Our results suggest that there is a lot of learning going on even beyond adolescence, even well into adulthood. Much later than anyone suspected,” says Hartshorne. “So we need to adjust our theories of learning to explain the typical stuff like ‘how does a four-year-old become a five-year-old?” but also ‘how does that four-year-old eventually become a twenty-five-year-old?” It just shows how much we don’t know.”

A new set of questions

Ulrich Mayr, a psychologist at the University of Oregon who studies aging, says that he’s not surprised by the findings, and they fit with other work that show peaks in particular disciplines. “There’s old work showing age trajectories in different academic disciplines. They showed that poets peak early but novelists peak later, for example,” he says. “More and more, we are seeing that certain aspects of abilities decline as we age and then others may get better. The general idea behind the old fluid/crystallized distinction has always been that every function basically marries those two different forces in some way. So you will see that some things will go up and some things will go down as you get older. But we don’t really understand why.”

Germine and Hartshorne agree. Germine says she thinks this work spurs new lines of inquiry, as well as reassuring us that aging isn’t all decline and doom.

“What we’ve done is open up lots and lots of questions. We’ve shown that the framework that we’ve been working with for decades doesn’t account for these sorts of patterns. So what kind of framework does?” says Germine. “We need to figure that out so we can have a clearer picture of what kinds of abilities are changing, why they are changing and what’s driving those changes. It’s going to take decades of work. But that work will help us map out what is happening across the lifespan in a much clearer way.”