Are fertility problems more common in men or women?

About the same: The estimate is 40-60% of infertility involves the male, so i would place it at about equal.

About equal: In a reproductive clinic it is most common to find contributory factors on both the male and female side of the subfertility problem. Testing is more exhaustive for women and most initial therapy includes treating the woman in some way. Ivf/icsi, for example is used to treat severe male factor, even if the woman is "normal", and that therapy involves extensive manipulation of the woman's system.

50/50: Agree with above. Roughly 40% male, 40% female, 10-15% combined, and 5-10% unexplained. My personal belief is unexplained is actually less common, not all couples all exhaustively tested. It may appear female infertility is more common, but it's just because there are more sub-categories we break it down to.