Birth order: First-borns are likelier to cheat, leave their partners if they marry fellow first-borns

"My parents were very strict with me. I was very mature and I took care of my four siblings - four boys ," says Mary [not her real name], a nurse. Growing up as a firstborn child, she felt bossy, confident and a motivated perfectionist. The now 50-year-old divorcee was the first daughter and first child raised under strict rules by her parents.

When she got married, Mary was 28 and said she was looking for someone responsible, who would take care of her and provide for her financially.