Extending the boundaries of research on adolescent development.

The mainstream psychology literature has historically failed to include individuals with nonheterosexual sexual orientations in its study of many areas, including adolescent development, sexual development, psychotherapy, couple relationships, aging, suicide, and substance abuse. The articles contained in this issue make clear that knowledge of human behavior will be more complete when research designs are expanded to reflect more accurately the diversity of sexual orientations and sexual expressions. Furthermore, when sexuality is categorized into a rigidly defined group like lesbian, gay, bisexual, and transgender (LGBT), it constrains the ways in which individuals might otherwise think about it. Knowledge of human behavior will be advanced more effectively when the field of psychology integrates people of diverse and complex sexualities into its research and studies the mediators and moderators associated with this diversity.