If you’re not happily, abstinently single, then at some point you’ll likely have a relationship. A 1994 report showed that 97 percent of people studied had at least one sexual partner at age 18 or later. Men tend to have more partners then women—in a 2005 study, heterosexual men 30 to 44 reported an average of six to eight lifetime sexual partners, where women in the same age range reported an average of four.

Having a sexual partner may seem like a low bar for “relationship,” but researchers have to find some clear marker in the squishy world of human connection. So many different kinds of relationship exist! For discussions of a few of those, see the hubs for monogamypolyamory , and swinging.

One thing’s clear—whether hetero or homosexual, relationships in the twenty-first century by no means equal marriage. The  2010 census figures for the United States show marriage rates at the lowest ever since record collection began more than 100 years ago. The United Kingdom reports the same thing. In both countries, people are waiting longer before walking down the aisle—U.K. statistics in 2009 put the average groom at almost 37 years old and his bride at nearly 34. Many nonmarried couples are choosing to live together instead.

Whether married, shacking up, or dating, most of us will be in a sexual relationship at one point. We’re social animals—we bond. One way we bond is sex.