Patricia East is a developmental psychologist who began her career working at an OB-GYN clinic in California. Thursday mornings at the clinic were reserved for pregnant teens, and when East arrived the waiting room would be packed with them, chair after chair of pregnant adolescents.
It was in this waiting room, explains, that she discovered her life's work — an accidental discovery that emerged from the small talk that staff at the clinic had with their young clients as they walked them back for checkups.
"The nurses and the doctors there would bring a teen back for her prenatal visit and they would say, 'Hey! Aren't you Maria's younger sister?' And the young woman would say, 'Yeah, I am!' And they would say to another patient, 'You know, haven't I seen you before?' And she would say, 'Yes, I was here for my older sister when she was pregnant.' "
Over and over East heard variations of this conversation, until it came to the point that when she saw a younger sibling sitting next to her sister in the waiting room an involuntary thought flashed across her mind.
For the rest of the story: http://www.npr.org/blogs/health/2013/04/29/179266284/Big-Siblings-Big-Influence-Some-Behaviors-Run-In-The-Family