Penile–vaginal penetration is the most common form of sexual intercourse. Studies indicate that most heterosexual couples engage in vaginal intercourse nearly every sexual encounter. The National Survey of Sexual Health and Behavior (NSSHB) reported in 2010 that vaginal intercourse is "the most prevalent sexual behavior among men and women of all ages and ethnicities". Clint E. Bruess et al. stated that it "is the most frequently studied behavior" and is "often the focus of sexuality education programming for youth." Weiten et al. said that it "is the most widely endorsed and practiced sexual act in our society."