I really enjoyed scrolling through more than half the article before finding the answer.

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Part of it seems to be the lifestyle choices that smarter people tend to make: namely, that they smoke at much lower rates. Similarly, smarter people are more likely to follow other healthy practices, have a better handle on their health care, and be less likely to work in a job that puts them at physical risk.

Socioeconomic status could play a role as well, perhaps allowing people to access better education and care

...

There are also genetic factors.

"""