Human Development

The Methods of Developmental Psychology

Many psychologists study human development. For some, charting the course of sensory and motor growth is of primary interest. For others, tracing the steps of language and conceptualization seems to hold the key to understanding what is unique about humans. Still other psychologists are interested in how animals and children learn, both to test their ideas about learning in general and to develop improved educational techniques.

This introductory module surveys the methods that developmental psychologists use in studying the child. As you read the text, try to answer the following questions.

The study of human development has interested proud parents throughout history. Many famous men, including Charles Darwin, James Mill, Jean Piaget, the Swiss child psychologist, and John B. Watson, the founder of behaviorism, have kept detailed records of the growth of their children, noting when the child first moved its head, rolled over, crawled, smiled, said a word clearly, cut teeth, walked, and so on. These diaries and journals plus our own records of experience with familiar children younger brothers or sisters or our own children give us what may be called observational data on child development. The first complete observation of a child was made by Milicent Shinn of Niles, California. She had been editor of the Overland Monthly and was the first woman to receive a Ph.D. degree from the University of California, Berkeley. She made daily records of the observations of her niece and published these records in The Biography of a Baby

Observational records are valuable, for they provide hunches and guesses about what is (and is not) important in any area of scientific interest. They also serve as a common-sense check on the conclusions reached in more controlled investigations.

Naturalistic observation is often the only way human development can be studied

Developmental psychology became scientific, like other forms of psychology, in the middle of the nineteenth century. However, developmental studies presented certain difficulties that were not found in other branches of psychology -- difficulties due to the nature of the subject. Many genuinely interesting problems, such as the effect of being deprived of mother love or the effect of malnutrition, could not be studied in controlled experiments. No one would starve babies of affection or food. Thus, although the experimental method has been employed when it was possible to make use of it without hurting infants and children, much of the investigation of human development has used other research methods.

Three types of comparisons provide data on the effect of developmental variables

Three major research methods will be studied in this section. They represent the most important kinds of investigation available to developmental psychologists today and include the cross-sectional study, the longitudinal study, and the cross-cultural study. A fourth method, the co-tvvin study, will also be mentioned briefly. These methods will be contrasted with the experimental method.

Cross Sectional Method
Compare groups that differ in age or background

The cross-sectional method of investigation often is used when the research aim is to compare developmental levels at various ages or backgrounds. Many children at different ages are studied in groups according to their age, and the results on the same sets of measures are compared for the groups. For example, the approximate age at which an infant can be expected to roll over, creep, crawl, pull himself up to a standing position, and walk unaided can be determined by observing the behavior of groups of children from birth until the age of about 15 months. If we, as investigators, study a group of one- month-old infants, another group of two-month olds, and a different group of babies at every month of age thereafter, we will have a cross-sectional research design. The results of the study might look like the chart in Figure 1, which shows the ages at which 50% to 75% of the children in each group demonstrated each skill.

Longitudinal Method.
Observe one group at different times

We could also have obtained our data using a longitudinal research design. In longitudinal studies, the researcher follows the same group of subjects. through the various stages of development that are measured. If we found group of newborn babies who were available for month-by-month measure -aments, we could complete the study with repeated observations of this one group.

Cross Cultural Studies
Compare groups from different cultures

Another important way of gathering data on human development is the cross- cultural method, which may be thought of as a special kind of cross-sectional study. People differ culturally to the extent that their customs, roles, and other learned behaviors that are passed on from generation to generation are differ- ent. It is often impossible to investigate the effects of certain variables, simply because they do not appear in our own society. Cradling, for example, is not practical in most Western countries. Yet the practice, in which infants are bound firmly to a board and kept from moving for most of their first year of life, is of interest to psychologists who study motor development. Dennis and Dennis (1940) found a way to study the effects of such enforced physical restriction. They compared the age of walking in Hopi children who had been cradled with those who had never been cradled, and found that the average age of walking was not affected by cradling.

A cross-sectional study may also compare people from different backgrounds. If the reading ability of six-year-olds were measured in low, middle, and high-income families, one would have a "cross-section" of reading ability at that age for the various income groups in a community.

Co-Twin Studies
Differences between identical twins are not caused by heredity

Developmental psychologists who want to rule out the effects of heredity in their investigations often use the co-twin study as a method of research. Co-twin studies typically compare identical twins who have been reared apart or who have been given different kinds of training. Hilgard's work in training digit memory in a pair of identical twins (Hilgard, 1933) was an example of a co-twin study. Hilgard trained one twin to remember digits in the first year, then trained the other twin in the second year, and compared their performance on frequent memory tests. He found that although both twins profited from the training, the twin trained later did better than the twin trained in the first year. Both twins lost their achievement gains after training was ended.

Cross-sectional, cross-cultural, and longitudinal studies may be thought of as "experiments done by nature." The investigator, for one reason or another, cannot manipulate any of the variables and must be content to identify important factors and observe relationships between them. However, true experimentation can often be done with children. Experiments with children, like all experiments, involve the manipulation of the independent variable, the measurement of a dependent variable, and the control of all other variables.

Hilgard's co-twin study is a good illustration of a controlled experiment with children. Hilgard trained digit memory in one twin in the first year, and trained the other twin in the second year. He then compared the performance of the twins. Hilgard gave the same training at different developmental stages. He manipulated the time at which training was given. Time of training was therefore the independent variable.

The dependent variable in an experiment is what is measured. What did Hilgard measure? He compared performance on digit memory tasks. Thus the dependent variable is performance on these tasks.

An important characteristic of experiments is that all other variables are held constant, as far as possible. Ideally, all experimental subjects should have identical experiences, apart from the differences the experimenter produces by manipulating the independent variable. Then we can be sure that the changes we measure in the dependent variable were produced by changes we made in the independent variable. Because controlling other variables is crucial, experimenters have often used co-twin studies in developmental psychology. Identical twins have identical heredity and usually a very similar environment. The use of twins gives control over important genetic variables that could be controlled no other way.


Now test yourself without looking back.

1. A team of psychologists wanted to study the learning process in infants. They used two groups of babies, all 3 or 4 days old. In one group, they presented a tone before they gave the baby a nipple to suck. In the other group, they simply presented the nipple to suck without the tone. After many presentations, they measured sucking responses following the tone for both groups and found a definite learning effect (Lipsitt and Kaye, 1964).

What is the independent variable?

What is the dependent variable?

3. Terman and Oden (1959) studied a group of gifted children from their early school years through middle age. They found that on the average these individuals talked early, walked early, were physically superior, and were typically social leaders as well. By the age of 35, many of them were listed in such books as American Men of Science and Who's Who.

4. Landauer and Whiting (1964) studied the height of adult men as a function of the degree of stressful treatment of infant boys. Their data were taken from observations of eighty pre-literate societies. They found that the average man in societies in which baby boys were ritually scarred, circumcised, or otherwise stressed, was taller than in societies where infants were not stressed. Landauer and Whiting conducted a study.


5. Gesell and his co-worker developed norms for four aspects of human growth: motor behavior, language behavior, adaptive behavior, and personal-social behavior. They studied large numbers of children in each age group to determine the approximate age at which each step in the growth process normally occurs.

6. A psychologist is interested in the effects of lack of attention on language development in babies. Which of the four research methods mentioned above would NOT be suitable for her investigation?




Reread the text. Then work these exercises.

Fill in the chart below with the name(s) of the investigator(s) of the appropriate study or studies. The studies you are to use are given below the chart.
Research Method Characteristics Studies Using This Method
a. Experimental The independent variable is manipulated, the dependent variable is measured, and all other variables are held constant.
b. Cross -sectional Groups at different stages or from different backgrounds are compared.
c. Longitudinal The same group of subjects b measured repeatedly and the measurements are compared.
d. Cross-cultural Groups from different societies are compared.