fundamental not only to psychologists’ understand-
ing of behavior but also to parents, schoolteachers, so-
cial workers, policymakers, and anyone else working
with other people. Because of its universality and its
implications for understanding behavior, the study of
individual differences is an essential part of any com-
plete scientific study of behavior.
See also: MILESTONES OF DEVELOPMENT; STAGES OF
DEVELOPMENT; TEMPERAMENT
Bibliography
Ainsworth, Mary, Mary Blehar, Everett Waters, and Sally Wall. Pat-
terns of Attachment. Hillsdale, NJ: Lawrence Erlbaum, 1978.
Bates, John, and Theodore Wachs, eds. Temperament: Individual
Differences at the Interface of Biology and Behavior. Washington,
DC: American Psychological Association, 1994.
Bowlby, John. Attachment and Loss, Vol. 1: Attachment. New York:
Basic, 1969.
Bowlby, John. Attachment and Loss, Vol. 3: Loss, Sadness, and Depres-
sion. New York: Basic, 1980.
Caspi, Avshalom. ‘‘Personality Development across the Life
Course.’’ In William Damon and Nancy Eisenberg eds., Hand-
book of Child Psychology, Vol. 3: Social, Emotional, and Personality
Development. New York: Wiley, 1998.
Damon, William. Social and Personality Development: Infancy through
Adolescence. New York: Norton, 1983.
Halverson, Charles, Jr., Geldolph Kohnstamm, and Roy Martin,
eds. The Developing Structure of Temperament and Personality
from Infancy to Adulthood. Hillsdale, NJ: Lawrence Erlbaum,
1994.
Harris, Judith. ‘‘Where Is the Child’s Environment? A Group So-
cialization Theory of Development.’’ Psychological Review 102
(1995):458–489.
Kohnstamm, Geldolph, Charles Halverson Jr., Ivan Mervielde,
and Valerie Havill, eds. Parental Descriptions of Child Personali-
ty: Developmental Antecedents to the Big Five? Mahwah, NJ: Law-
rence Erlbaum, 1998.
Mahler, Margaret, Fred Pine, and Anni Bergman. The Psychological
Birth of the Human Infant. New York: Basic, 1975.
McAdams, Dan. ‘‘Can Personality Change? Levels of Stability and
Growth in Personality across the Life Span.’’ In Todd Hea-
therton and Joel Weinberger eds., Can Personality Change?
Washington, DC: American Psychological Association, 1994.
Rothbart, Mary, and John Bates. ‘‘Temperament.’’ In William
Damon and Nancy Eisenberg eds., Handbook of Child Psycholo-
gy, Vol. 3: Social, Emotional, and Personality Development. New
York: Wiley, 1998.
Rubin, Kenneth, William Bukowski, and Jeffrey Parker. ‘‘Peer In-
teractions, Relationships, and Groups.’’ In William Damon
and Nancy Eisenberg eds., Handbook of Child Psychology, Vol.
3: Social, Emotional, and Personality Development. New York:
Wiley, 1998.
Scarr, Sandra. ‘‘The Development of Individual Differences in In-
telligence and Personality.’’ In Hayne Reese and Michael
Franzen eds., Biological and Neuropsychological Mechanisms:
Life-Span Developmental Psychology. Mahwah, NJ: Lawrence
Erlbaum, 1997.
Sperling, Michael, and William Berman, eds. Attachment in Adults:
Clinical and Developmental Perspectives. New York: Guilford
Press, 1994.
Wiggins, Jerry, ed. The Five Factor Model of Personality: Theoretical
Perspectives. New York: Guilford Press, 1996.
Brady ReynoldsPETS
Pets are a ready source of companionship, comfort,
and unconditional love for children, and they con-
tribute to a child’s emotional and physical well-being.
As children care for pets, they learn about responsi-
bility, friendship, sharing, and empathy toward both
animals and humans. Studies have shown that pets
also have a therapeutic value. Some hospitals have
pet-assisted therapy programs through which com-
panion animals, like dogs, visit young patients and
comfort them through the soothing effects of petting
and holding a soft, warm animal.
Selecting a pet takes careful consideration. Some
families want a pet to share in all family activities.
Some want to avoid animals with fur because of family
allergies. Others prefer caged animals like gerbils,
fish, or reptiles because they require less space and at-
tention. Whatever the pet choice, it should reflect a
family’s needs, income, living space, and lifestyle.See also: PERSONALITY DEVELOPMENT; SOCIAL
DEVELOPMENTBibliography
Beck, Alan, and Aaron Katcher. Between Pets and People: The Impor-
tance of Animal Companionship. West Lafayette, IN: Purdue
University Press, 1996.
‘‘How Pets Help People.’’ In the Humane Society of the United
States [web site]. Available from http://www.hsus.org/
programs/companion/pets_help_people.html; INTERNET.
‘‘Selecting a Proper Pet: Practical Advice and Considerations.’’ In
the American Veterinary Medical Association [web site]. Avail-
able from http://www.avma.org/care4pets/avmabuy.htm; IN-
TERNET.
Patricia OhlenrothPHENOTYPE
Phenotypes are the physical characteristics, such as
eye color, displayed by an individual. Phenotype,
often used in relation to specific variable characteris-
tics or disease states manifested by an individual, con-
tains no information about the underlying genetic
determinants of the characteristics. The genotype
contains the genetic information that is used to deter-
mine the range of potential phenotypes. Within this
range, the environment and other nongenetic factors
determine the final phenotype. For example, cystic fi-
brosis is a phenotype. Conversely, the description of
a mutation in the gene that causes this disease is aPHENOTYPE 305