In biological theory, gender development is the result of biological processes. Biological theorists are concerned with investigating the role of these biological processes and their role in forming the characteristics and attributes of men and women.
Chromosomes and their influence is a major area of interest for biologists in the field of gender development. Men and women have different chromosomes -- women have two X chromosomes while men have one X and one Y. Shortly after conception, the Y chromosome in males begins to produce testosterone and other male sex hormones, known as androgens. These androgens cause the embryo to develop a penis and testes and have been shown to cause the male brain to develop differently. When these androgens are secreted in adulthood, they can lead to aggression, which is one way that biologists have explained gender development
Hormonal activity is another focus area for biologists interested in the study of gender development. Men and women produce sex hormones in different quantities that are responsible for causing specific bodily functions, like the menstrual cycle in women, and have been shown to have an effect on behaviour. Testosterone, for example, is the primary male hormone and has been linked to aggression, which are characteristics that are considered to be masculine. In contrast, oestrogen, the primary female hormone, has been shown to create greater deposits of fat around the hips and waist -- a feminine attribute associated with maternity and child-rearing.
Biologists have also used the structure of the brain to explain the gender differences between men and women. Although men and women use both the left and right lobes of the brain, the sexes have specialised in using different lobes. Men, for instance, tend to use the left brain functioning that is responsible for linear and logical thinking while women's use of the right brain emphasises imagination, creativity and intuitive thinking. In terms of gender, these complement existing social stereotypes and ideals of male and female personality traits.
When it comes to human and animal studies on chromosomes and hormones, the biological theory of gender development is well-supported. A 1990 study by psychology professor David Buss, et al., at the University of Michigan, highlighted the importance of evolutionary processes in gender development. The research, titled "Evolutionary Personality Psychology," found that men highlighted the importance of youth and beauty while women searched for wealth and status.