Americans are less sexually active than in the past, and this decline is being led by the young.

In the early 2000s, about 73 percent of adults between the ages of 18 and 30 had sex at least twice a month. That fell to 66 percent in the period from 2014 to 2016, according to an analysis of US data.

Sex is also down among teenagers. Earlier this year, the Centers for Disease Control and Prevention reported a decline in the share of high school students who said they ever had sex: from 47 percent in 2005 to 41 percent in 2015. Sexual activity among teenagers fell the most between 2013 and 2015, about the same time that sex took a real dip among 18- to 30-year-old adults.

While the cause is not known for sure, social scientists have offered a few explanations. The younger generation tend to be more cautious.  There are growing concerns among women about the risk of a sexual encounter turning into an assault.  And a poor economy with little job security has made young adults increasingly likely to seek shelter with a parent than to live on their own or enter into marriage. Other say increased time spent online is a factor.