While there are many causes of male hair loss (plus numerous other factors that accelerate it), 95 % of men need look no further than their genes to find the culprit. The overwhelming odds are that if you're beginning to loose hair; then it's down to androgenetic alopecia. You don't need a scientist to tell you that you're more likely to loose your hair if someone in your family is bald, but it's surprising to find that the genes responsible for hair loss are as readily carried by men as by women (which debunks the myth about hair loss being a maternally inherited trait).
With age genetically susceptible males produce higher levels of an enzyme called 5-alpha reductase, which converts the hormone testosterone into dihydrotestosterone (better known as DHT). It's the DHT that's directly responsible for hair loss. Over time the DHT shrinks the hair follicles so that hairs become finer, and eventually the follicle may become so 'blocked' that hair is unable to grow (although the follicle may still be 'alive'). At the same time the DHT shortens the hair's growing phase, which in turn lengthens the shedding phase. The result is that more hair is being lost than grown, and what is grown is thin and weak.
If this were the only cause of hair loss then it would follow that; if you are able to inhibit DHT, then no one need go bald. Unfortunately this isn't the case, so it's obviously much more complex than at first thought. Similarly science has yet to fathom out why hair on the front and top of the head is more susceptible to DHT than that at the back and sides.
Other known causes of hair loss include:
Alopecia areata; an autoimmune condition where the body turns on its' own hair follicles
Telogen effluvium; where hair loss is a result of the body undergoing physical stress or mental stress
A symptom of underlying diseases such as diabetes, lupus or thyroid disorders
Medicinal side effect to drugs used to treat; high blood pressure (anticoagulants and beta-blockers) depression (Prozac), cancer, thyroid problems, stomach ulcers (Zantac), gout (Loporin, Zyloprim) and a number of anti-arthritic drugs. The above list is by no means complete, so if in doubt it's best to consult your health care professional (who may be able to provide an alternative.