A river begins high in the mountains: one channel carries more water, another less. In the same way, a person's social position sets the course of their life, but it doesn't carve the brain directly; it does so through tributaries: habits, blood pressure, blood sugar. Scientists measured these tributaries in 36,878 people to find out which ones carry away the most years.
For each participant, brain age was determined from MRI scans. A neural network learned to guess age from brain structure, and then its prediction was compared with chronological age. If the brain looked older, the river was flowing faster. The difference between the poorest and the most affluent was about four months, and this is not a coincidence but a stable gradient.
Next, the researchers broke down the link between a person's position and brain age into eleven factors, from cholesterol to how often they meet friends. Smoking turned out to be the widest tributary: it carried 0.022 years of difference for income and 0.016 years for neighborhood. It was followed by glycated hemoglobin, which reflects average blood sugar levels, and systolic blood pressure.
Alcohol behaved like a stream that flows first one way, then the other: in people with low income it seemed to slow aging, while in disadvantaged neighborhoods it accelerated it. This suggests that income, education, and neighborhood cannot be lumped into a single bag: each indicator has its own geography.
Let's return to the river. Previously, it was only clear that some people's brains age faster than others, but it wasn't clear which tributaries feed that difference. Now it's clear that the main tributary is smoking. So if we dam it, the river of inequality will carry away fewer years. Prevention programs aimed at quitting smoking and controlling blood sugar and blood pressure can narrow the channel through which poverty accelerates aging.
The average gap in brain age among participants was about 3.4 years on T1-weighted images, which is comparable to the effect of a decade of smoking.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.