Aja is worried that as her husband approaches his fortieth birthday he will become depressed and unhappy with his life and begin to seek other women to compensate for these feelings. studies cited in your text indicate that very few men go through this sort of:

The correct answer is midlife crisis.
It refers to a theory that people (usually men) start behaving differently once they come to a certain age. According to the theory, people will often buy a new car, change the way they look, and even start cheating on their partners with younger people because they feel as if they are reaching the second part of their lives and they are no longer as young and desirable. However, research shows that not that many men actually go through this phase.