Causes of Baldness

The most noticeable part of human hair is the hair on the head, more prominent in some than others, which is more dense than most hair found elsewhere on the body. The average human head has about 100,000 hair follicles. Each follicle can grow about 20 individual hairs in a person's lifetime. Average normal hair loss is about 100 strands a day.

Incidence of pattern baldness varies from population to population based on genetic background. Environmental factors do not seem to affect this type of baldness greatly. One large scale study in Maryborough, in central Victoria (Australia) showed the prevalence of mid-frontal hair loss increases with age and affects 57% of women and 73.5% of men aged 80 and over.

Male pattern baldness is characterized by hair receding from the lateral sides of the forehead, known as "receding hairline" or "receding brow." An additional bald patch may develop on top (vertex). The trigger for this type of baldness (called androgenetic alopecia) is DHT, a powerful sex hormone that adversely affects the hair and prostate.

The mechanism by which DHT accomplishes this is not yet understood. In genetically-prone scalps, DHT initiates a process of follicular miniaturization. Through the process of follicular miniaturization, hair shaft width is progressively decreased until scalp hair resembles fragile vellus hair or "peach fuzz" or else becomes non-existent. Onset of hair loss sometimes begins as early as end of puberty, and is mostly genetically determined. Male pattern baldness is classified on the Hamilton-Norwood scale I-VIII.

It was previously believed that baldness was inherited from a person's maternal grandfather. While there is some basis for this belief, both parents contribute to their offspring's likelihood of hair loss. Most likely, inheritance is technically "autosomal dominant with mixed penetrance".
There are several other kinds of baldness:

Traction alopecia is most commonly found in people with ponytails or cornrows who pull on their hair with excessive force. Wearing a hat shouldn't generally cause this, though it is a good idea to let your scalp breathe for 7 hours a day.

Traumas such as chemotherapy, childbirth, major surgery, poisoning, and severe stress may cause a hair loss condition known as telogen effluvium.

Some mycotic infections can cause massive hair loss.

Alopecia areata is an autoimmune disorder also known as "spot baldness" that can result in hair loss ranging from just one location (Alopecia areata monolocularis) to every hair on the entire body (Alopecia areata universalis).

Localized or diffuse hair loss may also occur in cicatricial alopecia (lupus erythematosus, lichen plano pilaris, folliculitis decalvans, central centrifugal cicatricial alopecia, postmenopausal frontal fibrosing alopecia, etc.). Tumours and skin outgrowths also induce localized baldness (sebaceous nevus, basal cell carcinoma, squamous cell carcinoma).

Hypothyroidism can cause hair loss, especially thinning of the outer third of the eyebrows

Temporary loss of hair can occur in areas where sebaceous cysts are present for considerable duration; normally one to several weeks in length.