Identical twins Bobby and Billy have a family history of depression. They learned in class that there may be genetic predispositions to depression. In other words, it can run in families.
Bobby's life is fast-paced. He has a difficult job, travels a lot, and has little time for relaxation. Billy's life is relaxed with a career he loves and plenty of time for recreational activities.
Around their 30th birthdays, Bobby began showing signs of depression, and by the time they turn 40, Bobby has been diagnosed with depression. Billy has never shown any signs of the disease.
Which of the following approaches may explain why Bobby has depression and Billy does not?