408 CHAPTER 10
the past month. And other than those over 70 years, 40 to 72 percent of females did so in
the past year, with the highest percentages in 18- to 49-year-olds (Herbenick et al., 2010).
The highest rates of vaginal intercourse were reported for both men and women across
age groups associated with reproductive years. However, across the same age groups, a signif-
icant number of individuals also reported solo masturbation, masturbation with a partner, oral
sex, and anal sex during the previous year (Herbenick et al., 2010). As noted by the research-
ers, sexual behavior during the reproductive years is not solely for reproductive purposes.
For most recent vaginal intercourse, 24.7 percent of adult men and 21.8 percent of
adult women reported use of a condom. In adolescent males and females, the rates were
79.6 percent and 70.2 percent, respectively. Condom use appears to drop off significantly
in early adulthood. Across all cohorts, condom use was more frequent when intercourse
took place with a casual partner as compared to a relationship partner (Reece et al., 2010).
Overall, the highest rates were reported during adolescence, were higher for men than
for women, and higher among black and Hispanic individuals (Reece et al., 2010).
EXPLAINING THE SURVEY FINDINGS Why are men so much more sexually active than
women, both before and during marriage? It may be in their genes. Evolutionary theory
emphasizes that organisms will do what they must to maximize their chances of pass-
ing on their genetic material in their offspring, and that process is different for men and
women. Robert Trivers proposed a theory of parental investment to explain the different sex-
ual behavior of men and women (Trivers, 1972). Males of many species, including humans,
do not have to invest a lot of time or effort into impregnating a female, so they are bet-
ter off—genetically speaking—when they seek many sexual encounters with many sexual
partners. Females, on the other hand, invest much more time and effort in reproducing:
the pregnancy, feeding the infants, and so on. So females are better off being more selective
about the males they choose for sex.
In real-life terms, this translates into men preferring women who are younger, pret-
tier (immediate sexual attraction being the big draw), and therefore are likely to produce
healthy, attractive offspring. Women are more likely to prefer men who are older (which
means they will likely have more income and resources), hard workers, and loyal (Buss,
1989, 2007; Buss & Schmitt, 1993, 2011). Men are much more likely than women to have
multiple sexual partners, even well into middle age, while women are more likely to have
fewer partners over their lifetime (McBurney et al., 2005; Schutzwohl et al., 2009). This is
true for both heterosexual and homosexual men and women (Peplau & Fingerhut, 2007).
Men even think about sex differently, having more sexual fantasies than women and of a
greater variety (Okami & Shackelford, 2001), as well as simply thinking about sex more
often than women (Laumann et al., 1994). There is some evidence, however, that differ-
ences in mate preferences are much smaller in cultures or social groups in which there is
greater gender equality, suggesting that many of the differences in mate preferences may
be related to differences in gender roles rather than just biology (Zentner & Mitura, 2012).
Surveys of sexual behavior highlight the fact that age is not necessarily a barrier to
being sexually active. One survey of more than 3,000 people aged 57 to 85 found that many
people are sexually active well into their 80s (Lindau et al., 2007). In this survey, the most
common barriers to sexual activity were health problems or lack of a partner rather than a
lack of desire. Another survey of 1,939 Spanish people over the age of 65 found that just over
62 percent of men and 37 percent of women were sexually active, although the incidence
of sexual inactivity did increase with age, with reasons for sexual inactivity being nearly
equally distributed among “physically ill partner,” “being widowed,” and “lack of interest”
(Palacios-Ceña et al., 2011).
Of course, surveys have their problems, as stated earlier. One possible problem
might occur when asking the question, “At what age did you first have sex?” A study
(Sanders et al., 2010) reports that not everyone means the same thing by the words “have
sex” or “had sex.” In a sample of people 18 to 96 years old, 30 percent did not consider
oral sex to be sex. Many older men—nearly a fourth of those surveyed—did not consider
