The enduring characteristics of an individual's behavior, attitude, and feelings in .
everyday social situations make up personality. There are many influences on an .
individual's personality, including culture, genetics, and early family life (as an infant or young child). Certain studies have shown that people with certain personality characteristics are more suitable for specific occupations or special activities. Personality studies can also reveal the .
relationship between specific types of personalities and the incidence of disease, divorce, .
and stress-related problems. .
There are many different theories concerning the nature and development of .
personality and the causes of personality changes. The ancient Greek physician .
Hippocrates believed that people behave differently because they have a predominance of .
one type of bodily fluid, or humor. According to this theory, people with calm or passive .
personalities have one dominant humor, while impulsive and temperamental people have a .
different dominant humor. In the 20th century, other classification schemes were .
advanced. The German psychiatrist Ernst Kretschmer thought that personality was .
determined by the person's body type--such as plump, lean, or athletic. He suggested that .
short persons were more likely to be social, friendly, and lively. Both of these theories .
remain unproven. Major advances have been made through behavioral genetics, the field .
concerned with the relative influence of genetic and environmental factors on individual .
behavior. One approach is to study the personalities of identical (monozygotic) twins who .
have been separated since birth. Such individuals have the same genetic makeup but differ .
in the environments in which they are reared. In a related method of study, the .
personalities of individuals who are adopted from birth are compared to the personalities .
of their adoptive and biological parents. These individuals have genetic material different .
