Unfortunately, hormone effects and interactions can be so complex that even the best known hormones are not completely understood." (MacDougall) Hormones are most often discussed in connection with sexual behavior. But they are responsible for almost every reaction and action. When a person takes on too much stress, stress hormones are to blame and these hormones cause the behavior from that. Similarly during a phase of depression when a person is undergoing acute pessimism, the production of endorphins stopped or is lowered which is the really cause of depression. For this reasons, medication is given which balances chemical reactions in the body to normalize behavior. Human behavior is thus a sum total of different influences including heredity, hormones, environment and psychology. Dr. Leary has explained this in an apt manner when he said: The science of human behavior in the largest sense of the word behavior, the sense which includes all that human beings do, all that human beings have. In this sense of behavior it is the behavior of the whole, integrated personality which is under investigation....Psychology deals with the organism as a whole, as an integrated and orientated individual in contact with other individuals in a complex external environment, partly physical and partly social, in short, as a personality....The behavior of...

reduces to physiological facts and findings, in turn to those of the field of biology, then to those of bio-chemistry, then to chemistry in general and then, inevitably, to physics as the science of matter in motion." (p. 10-18)
Heredity on the other hand has often been touted as another major source of understanding human behavior but many scientists now feel that it alone cannot determine behavior or cause someone to behave in a certain manner. Francis Galton studied the connection of heredity with human behavior in early 1900s when the term genetics had not been coined yet. Galton found some correlation between heredity and criminal behavior and this sparked intense interest in the field. Researchers now maintain that while someone may be genetically predisposed to certain behavior, they are not likely to act till there are other influences fuelling that action. In other words, heredity is a weak source or cause in isolation but together with environment and hormones, it may influence a person's behavior.

References

Leary, Daniel B., Modern Psychology: Normal and Abnormal, Philadelphia and London, J.B. Lippincott Co., I928

Peter MacDougall, UBC archives. Accessed online 10th Jan, 2006 from, http://www.library.ubc.ca/archives/pdfs/432/THE432_1988_02_10.pdf