According to new research published by the National Bureau of Economic Research, decreasing life satisfaction over our thirties and into our forties is a nearly universal human experience. There is an antidote to all that unhappiness though: marriage. Multiple studies have shown that people who are married are able to stay much more cheerful through these difficult middle years.

I have a theory about why we see this relationship: sex. Sex makes people happy. Young people have more sex than middle-aged people. And middle-aged people who are married have more sex than those who are single.

For more on this, see my latest in The Daily Beast.