Dr. Janssen's research interests include the determinants of sexual desire and arousal, the effects of emotions on sexual response and behavior, and sexuality and relationships. A large part of his work focuses on a newly developed model of sexual response and behavior which, with its focus on individual differences in the propensity for sexual excitation and inhibition, has opened up a new research agenda with wide relevance. The model is being used and tested by researchers around the world in studies on sexual health topics as varied as sexual risk taking, sexual dysfunction, sexual compulsivity, and sexual aggression.

Studies in over 25,000 men and women have consistently shown that large individual differences exist in sexual excitation and inhibition proneness, and suggest that this variability is associated with, on the one end, a vulnerability to sexual dysfunction and, on the other end, an increased likelihood of engaging in risky sexual risk behaviors. Most studies related to sexual risk taking focus on risk for contracting sexual transmitted infections (STIs) and unprotected sexual activities. Dr Janssen's research conceptualizes sexual risk taking more broadly to include behaviors that may jeopardize other aspects of well-being (e.g., relationships, mood).

Dr. Janssen also studies the relationship between sexual and nonsexual emotions (e.g., love, sadness, anger) and how these, separately and in interaction, influence sexual functioning, behavior, and satisfaction, in individuals as well as in couples. In approaching this work, Dr Janssen applies questionnaire, psychophysiological, daily diary (using smart phone technology) as well as interview and focus group methods.