They wanted to know how social networks influence behavior. Most epidemiological models of the spread of sexually transmitted diseases use estimates of the average number of sexual partners a person has in a given population. For example, statistics might show that on average, women born after 1950 have ten partners in their lifetime. Traditional models would presume that their mating is more or less random. Laumann thinks this is a poor way to model the spread of sexually transmitted diseases.
People engage in different patterns of sexual activity at different times of their lives.
For instance, after a burst of experimentation in her teens and twenties, a woman might remain monogamous for 20 years, divorce, and have several sexual partners before returning to monogamy. Hence you might expect a different pattern of disease spread in a population where most women are under age 30 than in one where most women are older. Social and economic status also play a role. Sex between partners of the same age, ethnic group, and economic standing may be more openly negotiated, thereby lessening the risk of unprotected sex. So Laumann proposed a model of sexual behavior as something done by couples living in social networks, rather than by randomly acting individuals, as most epidemiological models presume.