They have the aptitude, but just aren't naturally interested in science, and eventually leave.

They want to have families and find it difficult to stay in fast-paced science careers.

They get fed up with the stressful lifestyle earlier than men do.

They don't have natural aptitude for technical professions and can't cut it at the higher levels.

They ARE in science, just not in typical roles (i.e., assistant professorships, instructorships, etc.).

They are, consciously or unconsciously, encouraged to leave.

Other: