Developmental psychology.pdf

60 The Human Organism
The study of human development concerns the ways in which people change
with the passage of time. It emphasizes a pattern of changes throughout the life span,
often in three broad areas: physical, social, and cognitive. We consider each of these
areas and then an issue underlying all of them: the influences of heredity and environment.
The chapter concludes with a discussion of the full life cycle and a comment on
individual differences.
PHYSICAL DEVELOPMENT
The Iowa orphans were housed in the fashion of that day, according to age, sex, and
ability. Infants up to six months old remained in standard cribs with no toys or mobiles.
Instead, a protective covering on the sides of the crib greatly limited visual stimulation.
At six months of age, they were moved to dormitories under the care of two nurses,
wliose responsibilities were largely limited to cleaning, feeding, and dressing. Afterward,
at two years of age, they were graduated to cottages, and when they were six
years old, if appropriate, they began formal schooling.
Psychological services were introduced at the orphanage in 19,32 in the person
of Harold M. Skeels, employed by the Board of Control of State Institutions. His appointment
recognized that psychology might make a contribution in the area of child
welfare, and almost immediately he noticed by chance two malnourished, frail, and
"pitiful little creatures." These infant girls, 13 and 16 months of age, should have been
in the dormitories, but they behaved more like the older babies in the cribs. Sad and
inactive, they spent their days rocking and whining, although no physical defect could
be discovered (Skeels, 1966).
Figure 3.1
Conception. Taken with a powerful
electron microscope, this photograph
shows the sperm of a sea urchin
making contact with the egg.
Figure 3.2
Human Chromosomes. They do not
appear in pairs but can be matched
in this fashion.
Biological Influences
The behavior of these children was partly a function of their genetic endowment and,
like all of us, each child began life in a very small way, as a single cell. This fertilized
cell, resulting from the union of the father's sperm and the mother's egg, is but half
the size of the dot over this "i," yet all of the inborn influences on our physical, mental,
and even social development are set at this miraculous moment of conception (Figure
3.1).
As we leave these infants to consider the structures and processes behind all
of our origins, we must realize that no further hereditary influences are possible during
the rest of our lives. Our entire heredity, the sum of characteristics and potentialities
biologically transmitted from our parents, is contained in this one fertilized cell.
Chromosomes and Genes Within the fertilized cell are complex organizations of
chemical materials that contain the determiners of our biological inheritance. First
seen by scientists as colored strands in the stained cells, they were named chromosomes,
meaning "colored bodies." Microscopic studies have shown that there are 46
chromosomes in every human cell, and these can be arranged on the basis of size and
form into 23 pairs.
One pair consists of the sex chromosomes, X and Y, which result in the development
of a male or female. The male is XY, receiving an X from the mother and
a Ffrom the father, whereas the female receives an X from each parent. The two little
girls each had an XX combination (Figure 3.2).
Within the chromosomes are even more fundamental determiners of heredity,
called genes, which contain the basic blueprint that specifically directs the development
of most physical characteristics and certain behavioral traits. Their numbers are still
a matter of guesswork, with estimates varying from 1,000 to 100,000 per chromosome.
During prenatal life, as the first cell successively multiplies, the complete genetic code
is passed on to each of the resulting cells. All cells in the human body thus contain the
same inheritance with one exception, theTeproductive cells (Figure 3.3).