The document summarizes some key biological factors involved in human sexuality. It discusses how sexual behavior in humans serves purposes beyond reproduction, shaped by both biological and cultural influences. While culture has powerful effects, biological factors related to sexual arousal and response remain fundamental to understanding human sexuality. It then describes the processes of sexual differentiation into male and female, including the roles of sex chromosomes and genes in developing either testes or ovaries which influence further physical and neurological sexual differentiation. Areas of overlap between male and female development are also noted.
1. • Biological Factors in Human Sexuality
• Journal article by John Bancroft; The Journal of Sex Research, Vol. 39, 2002
Biological factors in human sexuality.
by John Bancroft
In most species sexual behavior principally serves the purposes of reproduction. In
humans, and in some other primates, other purposes for sexual behavior in addition to
reproduction have evolved. In the human such purposes have been shaped and
influenced by cultural factors, so that human sexuality has been expressed in many
different ways, varying across cultures and over history. In spite of these powerful
cultural influences, biological factors involved in sexual arousal and response remain
fundamental to human sexual experience and need to be taken into account in our
attempts to understand and explain the complexities and problems as well as the
positive aspects of the human sexual condition. The relevant literature is vast and this
paper makes no attempt to provide a comprehensive review; rather, it should be
considered an overview of a number of key concepts relevant to understanding the role
of biology and its interaction with culture in shaping human sexuality.
SEXUAL DIFFERENTIATION
Biological differentiation into male and female makes sexual reproduction possible.
Males have two sex chromosomes, an X and a Y; females have two X chromosomes.
Sexual differentiation is determined by the presence or absence of the Y chromosome.
If there is no Y chromosome, development is along female lines. A specific part of the
Y chromosome, the SRY (sex-determining gene region of the Y chromosome) is
responsible for male differentiation (Haqq & Donahoe, 1998). This results in the
differentiation of the primitive gonad into a testis rather than an ovary during early
fetal development. The resulting testis produces not only testosterone but also
Mullerian Inhibiting Factor (MIF) which actively suppresses any further development
of those parts of the embryo which would otherwise develop into internal female
reproductive organs. The testosterone stimulates development of male genitalia and
reproductive organs: It also has organizing effects on the central nervous system which
are not well understood but which increase the likelihood of male patterns of behavior
after birth (Collaer & Hines, 1995). The SRY, or possibly other parts of the Y
chromosome, may have other gender differentiating effects on the central nervous
system which are not dependent on testosterone, but these have not as yet been
identified.
The bisexual potential of the embryo allows for selection of development along either
male or female lines (Beach, 1976). In some instances, as with the gonad becoming
either a testis or an ovary, or the female reproductive tract being actively suppressed
during male development, there is clear differentiation. In other cases, such as the
nipples of the male or the clitoris of the female, there is no biological need to suppress
their development, even though they play no direct role in sexual reproduction. This
undifferentiated overlap between male and female may apply to other structures or
functions, such as orgasm in the female. Whereas orgasm is essential for the male's
contribution to reproduction, orgasm in the female has no obvious reproductive
purpose. It has been argued that female orgasm has positive benefits which would
justify its existence on evolutionary grounds, but it seems more likely that its benefits
to women are a "biological bonus" resulting from the absence of active suppression of
its development (Lloyd, 1993; Symons, 1979). The role that female orgasm plays in