I have a few theories on why there seems to be a high number of shaved heads in gyms:1. As others have already pointed out, people who are taking time to work out might be a little more appearance-conscious than the general population2. Guys who are developing MPB might have a little more incentive to stay in shape. It's one thing to be a bit overweight or to even be bald, but the two together are generally not considered to be a positive thing3. Some of the body-builder types might like to look a little tougher4. Steroid use can cause hair loss. Hopefully this only applies to a small minority.
I believe there is also a causal link between high testosterone and MPB -- working out = higher testosterone.or perhaps higher testosterone individuals are more prone to work out as well? YMMV
I don't really notice...then again I'm not really looking...there was this woman on the treadmill the other day...