Who is Laurence Steinberg from High Tide in Tucson: Essays from Now or Never and what is their importance?

Kingsolver references a study done by psychologist, Laurence Steinberg, in High Tide in Tucson: Essays from Now or Never. The study she mentions focused on the impact of children on their parents' mental health. Research found that the most trying time for parents was during the terrible twos and adolescence.