Fisher, like many other experts, attributes the sex decline to a decline in couplehood among young people. For a quarter century, fewer people have been marrying, and those who do have been marrying later. At first, many observers figured that the decline in marriage was explained by an increase in unmarried cohabitation — yet the share of people living together hasn’t risen enough to offset the decline in marriage: About 60 percent of adults under age 35 now live without a spouse or a partner. One in three adults in this age range live with their parents, making that the most common living arrangement for the cohort. People who live with a romantic partner tend to have sex more than those who don’t — and living with your parents is obviously bad for your sex life. But this doesn’t explain why young people are partnering up less to begin with.