When we talk of personality, we don't mean that a person has charm, a positive
attitude toward life, a smiling face, or is a finalist for "Happiest and Friendliest"
in this year's Miss America contest. When psychologists talk of personality, they
mean a dynamic concept describing the growth and development of a person's
whole psychological system. Rather than looking at parts of the person, personal-
ity looks at some aggregate whole that is greater than the sum of the parts.
   The most frequently used definition of personality was produced by Gordon
Allport more than 60 years ago. He said personality is "the dynamic organization
within the individual of those psychophysical systems that determine his unique
adjustments to his environment."² For our purposes, you should think of person-
ality as the sum total of ways in which an individual reacts to and interacts with
others. It is most often described in terms of measurable traits that a person exhibits.

PERSONALITY DETERMINANTS

An early argument in personality research was whether an individual's personal-
ity was the result of heredity or of environment. Was the personality predeter-
mined at birth, or was it the result of the individual's interaction with his or her
environment? Clearly, there is no simple answer. Personality appears to be a re-
sult of both influences. In addition, today we recognize a third factor-the situa-
tion. Thus, an adult's personality is now generally considered to be made up of
both hereditary and environmental factors, moderated by situational conditions.

Heredity   Heredity refers to those factors that were determined at conception.
Physical stature, facial attractiveness, gender, temperament, muscle composition
and reflexes, energy level, and biological rhythms are characteristics that are gen-
erally considered to be either completely or substantially influenced by who your
parents were, that is, by their biological, physiological, and inherent psycholog-
ical makeup. The heredity approach argues that the ultimate explanation of an
individual's personality is the molecular structure of the genes, located in the
chromosomes.
   Three different streams of research lend some credibility to the argument
that heredity plays an important part in determining an individual's personality.
The first looks at the genetic underpinnings of human behavior and tempera-
ment among young children. The second addresses the study of twins who were
separated at birth. The third examines the consistency in job satisfaction over
time and across situations.
   Recent studies of young children lend strong support to the power of hered-
ity.³ Evidence demonstrates that traits such as shyness, fear, and distress are most
likely caused by inherited genetic characteristics. This finding suggests that some
personality traits may be built into the same genetic code that affects factors such
as height and hair color.
   Researchers have studied more than 100 sets of identical twins who were
separated at birth and raised separately.⁴ If heredity played little or no part in de-
termining personality, you would expect to find few similarities between the sep-
arated twins. But the researchers found a lot in common. For almost every behav-
ioral trait, a significant part of the variation between the twins turned out to be
associated with genetic factors. For instance, one set of twins who had been sepa-
rated for 39 years and raised 45 miles apart were found to drive the same model
and color car, chain-smoked the same brand of cigarette, owned dogs with the
same name, and regularly vacationed within three blocks of each other in a
beach community 1,500 miles away. Researchers have found that genetics ac-