What your DNA – and childhood – reveal about midlife decrease in IQ

The pace of your DNA aging may help predict whether your brain stays sharp or declines by the time you reach your 50s, and childhood socioeconomic background can make that decline more pronounced, according to researchers at the USC Dornsife College of Letters, Arts and Sciences and the University of Virginia.

In a decades-long study of hundreds of twins, the team found that those whose DNA showed signs of accelerated biological aging were more likely to experience IQ decline from childhood to midlife. The drop was steepest among those raised in lower-income households.

The study, published in the journal Aging and funded by the National Institute on Aging, suggests that biology and environment work together to shape long-term brain health.

Biological aging is more than just birthdays

We commonly measure age in birthdays, but scientists also track “biological age” to evaluate how well the body is holding up.

One way to measure biological age is by looking at DNA methylation, the presence of tiny chemical tags on the DNA strand that turn genes on and off. These tags shift over time depending on lifestyle, health and stress, offering a molecular snapshot of whether someone’s body is aging faster or slower than average.

“Your DNA carries a timestamp of how your body is doing,” said Christopher Beam, associate professor of psychology and gerontology at USC Dornsife and a co-author of the study. “We wanted to know if that biological clock could also tell us something about how well cognitive functioning holds up from childhood to midlife.”

Study taps rich data from long-standing twin study

The researchers drew on the Louisville Twin Study, one of the longest-running projects of its kind. Beginning in the 1950s, the study has tracked children’s IQs and recorded details about their family backgrounds. Decades later, many of those same twins — now in their 40s, 50s and 60s — returned for follow-up testing.

For this study, the team focused on 287 twins who had completed IQ tests as children and again as adults. Participants also gave blood samples, which scientists analyzed to measure DNA methylation and estimate biological age.

Because the identical twins share the same DNA and grew up in the same households, comparing them allowed the researchers to rule out many genetic and family-level factors and zero in on whether accelerated biological aging was linked to changes in intelligence.

Faster biological aging means steeper IQ declines

Across the sample group, people with faster biological aging showed greater declines in IQ from childhood to midlife. And for those raised in lower-income households, the effect was most pronounced.

“Childhood disadvantage seems to leave a fingerprint,” Beam said. “It may set people up to be more sensitive to the effects of biological aging on the brain decades later.”

Deborah Finkel, professor (research) of psychology and a research scientist at the USC Dornsife Center for Economic and Social Research, said the twin design proved crucial. “Because we were looking at identical twins, we could see that when one twin was biologically older, that same twin tended to have a sharper drop in IQ,” she explained. “That suggests the relationship is more than just family background or genetics.”

The team also compared two types of “DNA methylation clocks.” First-generation clocks (Horvath, Hannum, and Horvath Skin & Blood) estimate chronological age, while second-generation clocks (GrimAge and PhenoAge) track health risks and lifespan. Only the second-generation methods predicted lifetime IQ decline.

Twin study points to early warning signs

Cognitive decline, even if subtle, can affect quality of life long before old age. While the study doesn’t prove cause and effect, it points to a pathway linking early-life environment, biological aging and brain health.

The researchers also found that smoking partly explained the connection between accelerated biological aging and IQ decline. Yet even after accounting for it, the trend persisted, suggesting that both lifestyle choices and childhood circumstances shape long-term brain health.

Think of it like a warning light on your car’s dashboard.

The findings could eventually help doctors identify who is most at risk for problems later in life. If scientists can track biological aging in young or middle-aged adults, they might be able to intervene earlier — with lifestyle changes, education or medical care — to protect long-term cognitive health.

“Think of it like a warning light on your car’s dashboard,” said Beam. “If we can spot accelerated aging early enough, there may be ways to head off some of the negative outcomes we see later.”

For the average person, the study underscores two important lessons. First, the conditions we grow up in matter, and poverty in particular can leave lasting marks on health. Second, lifestyle choices such as smoking, drinking and poor diet can accelerate biological aging. That means there’s at least some room to take control.

“The pace of biological aging isn’t fixed,” said Sophie Bell, a doctoral student researcher at the University of Virginia and the study’s first author. “It’s dynamic and can be influenced by what we do and what we are exposed to throughout the lifespan. Avoiding smoking, managing stress, and staying physically active all make a difference.”

Twin study offers a bigger picture of biological aging

The study is one of the first to connect biological aging with changes in intelligence across such an extended time span. It also highlights the need to consider social factors, not just biology, in understanding brain health.

“This research shows the story of aging is written by both our DNA and our environment,” said Beam. “The two are intertwined, and we need to think about both if we want to promote healthy aging.”

DNA methylation tests aren’t part of routine medical care as of yet. But as the science advances, they could one day become a powerful tool for measuring how well we’re really aging on the inside.

About the study

In addition to Beam, Finkel and Bell, study authors include Alyssa Kam of USC Dornsife; Ebrahim Zandi of Keck School of Medicine of USC; Emily Andrews and Eric Turkheimer of the University of Virginia; and Jonathan Becker and Deborah Davis of the University of Louisville School of Medicine.

Funding was provided by National Institute on Aging grant R01AG063949.

Editor’s Note: Darrin S. Joy contributed to this article.