Effects of Heredity and Environment
on individual Differences

Most psychologists acknowledge that both heredity and environment are influential; the argument is over which factor is predominant

The nature-nurture issue first became a controversy in the 1920's and the 1930's. Some psychologists felt that heredity was the primary, if not the sole, factor in the development of an individual. Others felt environment was the critical determining factor. As we saw in Unit One, this controversy is still unresolved. Still, most psychologists agree that some degree of interaction between heredity (nature) and environment (nurture) determines both physical and psychological development. In this module we will consider the relation of both factors to intelligence and behavior disorder.

As you read the text, consider the following questions.

The interaction between heredity and environment can be studied by investigating families and persons who are related in various ways. Hereditary similarity is relatively well understood. A parent and child, for example, have 50% of their genes in common, but among siblings this may vary from 0 to 100% due to the random distribution of chromosomes. Identical twins have the same heredity; 100% of their chromosomes are alike.

Environmental differences cannot be stated in such concrete terms. No two persons experience an identical environment. Very similar environments are usually experienced by same-sexed twins, whether identical or fraternal. Different-sexed twins, siblings, and parents all experience somewhat different environments, although these environments may be superficially similar. Occasionally, identical twins are reared apart. Then we have two people with identical heredity but different environments. Studies of such situations are valuable in the analysis of the nature-nurture issue. When twins are reared apart, a higher degree of similarity between identical twins than between fraternal twins is assumed to indicate the part played by heredity.

Those who consider environment most important have an advantage; they can at least observe the environment directly

A study by Polansky et al. (1969) illustrates the difficulty in separating the relative effects of genetics and the environment. The researchers studied poor families in the southern Appalachian area. They found a positive correlation between the IQ's of the mother and children which would seem to be a hereditary factor. But they also found a correlation between the mother's IQ and the adequacy of her child-caring. It may have been this latter environmental factor and not heredity which produced the first correlation.

The causes of many forms of mental illness are also thought to be hereditary. A great deal of study has been done on schizophrenia, which is a serious disorder. Using family histories, Book (1953) studied the occurrence of schizophrenia in three isolated areas in northern Sweden. He traced most of the cases to 31 ancestral pairs that formed one family pedigree. On the basis of this finding, Book suggested a hereditary basis for schizophrenia, with environmental factors modifying the severity of the condition.

Since heredity cannot be observed directly, its influence is more difficult to document

Another source of evidence pointing to hereditary factors in schizophrenia is provided by co-twin studies. A classic study of the incidence of schizophrenia (Kallman, 1946) used this method. Kallmann reported that when schizophrenia occurred in one of an identical set of twins reared apart, the co-twin also had the disease in 77.6% of the cases. The percentage was even higher for identical twins reared together in a similar environment. With fraternal twins the percentage was 14.7, which is very similar to the percentage for ordinary siblings. Although the study seems to indicate a large role played by heredity in the development of schizophrenia, this conclusion has been attacked on the grounds that many of the twins lived together for a considerable time before they were separated.

In reanalyzing some older data, Rosenthal (Rosenthal, 1959; Slater, 1953) separated the identical twins into groups in which both members had the disease and groups in which only one was affected. He found evidence for two types of schizophrenia─only one essentially based on heredity. For example, in 60% of the cases in which both twins had the disease, the family background showed schizophrenia. In less than 8% of the cases in which only one twin was affected was there any family history of schizophrenia. He concluded that there could be only two types of schizophrenia, one with and one without a hereditary component.

Thus most of the research is undertaken to validate hypotheses as to the influence of heredity

Most research on the nature-nurture question has been concerned with hereditary factors in intelligence. The most useful approach has been through correlations between siblings (including identical twins) or between parents and offspring. Studies comparing the IQ scores of identical twins reared together and apart have been particularly useful. Figure 18 gives the correlation coefficients for data collected in one study (Newman, Freeman, Holzinger, 1937). It showed that fraternal twins have a lower correlation coefficient than identical twins and also revealed a higher correlation for identical twins reared together than for identical twins reared apart This persisting similarity in identical twins indicates that heredity is indeed an important factor in determining intelligence but the lower correlation for twins reared apart indicates that environmental influences can change the hereditary potential.

IQ Correlation Coefficient
Fraternal Twins {reared together}
Identical Twins (reared apart)
Identical Twins (reared together)

Identical Twins

A recent study (McClearn, et al., 1997) on 110 identical twins 80 or more years old has yielded similar results. The similarity between 110 Swedish same-sex twin pairs with no cognitive, sensory, or motor impairment was greater than that among 130 fraternal same-sex twins. The estimates of inheritance of cognitive abilities was 62 percent and 55 percent for verbal ability. This is not to rule out environmental influences, which were judged to be about 40%. But it does seem clear that hereditary factors need to be taken into account.

The presence of environmental influences is further substantiated by evidence (McNemar, 1942) that fraternal twins are more alike in intelligence than are ordinary siblings. Environmental influences must be involved here. Fraternal twins are no more alike in heredity than ordinary siblings, but they would tend to have a more similar environment. Another study (Jones and Conrad, 1954) has shown that the correlation of intelligence scores is greater between siblings ten years old than between siblings four years old. This study indicates that the longer children live in similar environments, the more alike they become in IQ scores.

Today a commonly accepted view of intelligence is that heredity places a "ceiling," or upper limit, on an individual's IQ. An unstimulating environment, however, can prevent someone from reaching his upper limit. The individual's actual IQ, therefore, is determined by both his heredity and his environment.

Is early childhood stimulation crucial to the development of high ability children? There is evidence that during the first three years of life a high degree of synaptic connections is developed. Further, the brain appears more plastic and certain environments can produce more synaptic density. A book by John Bruer (reviewed by Mark Johnson), however, claims that the crucial first three years is a myth. He claims that the studies made are comparing normal child rearing with deprived children; so the effects shown are really due to deprivation, not to favorable stimulation.


Now test yourself without looking back.

1. The most widely accepted view of the "nature-nurture" issue is that most characteristics are determined by__________________________________

2. Which of the following might indicate the hereditary influence on schizophrenia?
a. If one fraternal twin has schizophrenia, his co-twin is more likely to also have the disease than another sibling.
b. If one identical twin has schizophrenia, his co-twin is more likely to have the disease than another sibling.
c. -(neither)

3. The schizophrenia correlation between identical twins reared apart has been found to be around .77. For those reared together it is approximately .90. This evidence supports the view that:
a. schizophrenia is environmentally produced.
b. schizophrenia is genetically determined.
c. both heredity and environment contribute to the occurrence of schizophrenia.
d. neither heredity nor environment is the sole cause of schizophrenia.

4. The most useful method for studying the nature-nurture issue is the___________________

5. Several researchers have found a high correlation in 10 scores between identical twins reared together. Give two possible explanations for this single fact in terms of the nature-nurture issue.



Identical twins have identical heredity. Fraternal twins are no more similar in heredity than ordinary siblings. Check the set of correlation coefficients that would indicate that a trait is hereditary.

Correlation Between IQ Scores
identical twins
fraternal twins

Correlation Between IQ Scores
identical twins
fraternal twins

Correlation Between IQ Scores
identical twins
fraternal twins

Siblings have an average of 50% of their genes in common. Fraternal twins have- an average of 50% of their genes in common and usually have a more similar environment than do siblings of different ages. Identical twins have the same genes and usually a very similar environment. Match the following sets of hypothetical correlations with the kind of causation they might support,
1) Heredity is the main factor_________
2) Environment is the main factor ________
3) Heredity and environment interact to produce the result_______

fraternal twins

siblings (reared apart)
fraternal twins (reared apart)
identical twins (reared aprt)

fraternal twins (reared together)
identical twins (reared apart)
identical twins (reared together)


If a trait were completely determined by heredity, the correlation' between scores for identical twins would be:

a, 0.0.
b. 0.25
c. 0.50.
d. 1.00.

______________________________________ 1

1 d

1) b
2) a
3) c

5 a

If a trait were completely determined by environment, the correlation between identical twins would be:
a. 0, because no two persons have the same environments.
b. 1.00, because identical twins are reared in the same environment.
c. highly positive, if they were reared together.
d. low, if they were reared in very different environments.

The percentage of similarity between both types of twins for
three diseases is shown below (recall that measles is contagious
whereas schizophrenia and cancer are not):
Identical fraternal
schizophrenia 86 15
measles 95 87
cancer 61 44
For which disease in the chart shown above is heredity most
likely to be a factor?____________________________________________8

How can you tell from the chart that heredity is not the only factor in the disease?________________________________6

For which disease is environment most likely to be a factor?__________________ _1

One study (Skodak, 1950) found that two unrelated children adopted into the same home correlated the same as natural siblings in intelligence. This is evidence for:

a. the advantage of high socioeconomic level for IQ.
b. environmental influence on IQ.
c. hereditary influence on IQ.
___________________________________________ 7

Another study (Skodak and Skeels, 1949) found that the IQ correla- tion between adopted children and natural mothers was .44. The correlation between the children's IQ's and the adoptive mothers' educational levels was .02. This is evidence for_________________________________4



1. measles
3. c, d
4. hereditary influence on intelligence
6. The correlation for identical twins is not 1.00.
7. b
8. schizophrenia


1. The current view of the nature-nurture issue is that:
a. every characteristic is determined solely by either heredity or environment.
b. the determination of most traits depends on an interaction between heredity and environment.
c. heredity is the controlling factor in most traits.
d. (none of these)

2.One study (Conrad and Jones, 1940) found the correlation in IQ scores between parents and their children to be .49. This could be evidence for the influence of:

a. heredity, if the correlation between unrelated adults and children were very high.
b. heredity, if the correlation between unrelated adults and children were very low.
c. environment, if the parents and children lived apart.
d. (none of these)

3. The correlation between IQ scores of identical twins reared together is greater than that for identical twins reared apart. What does this indicate?__________________________

4. Which of the following could indicate that both hereditary and environmental factors influence schizophrenia?
a. Every time an identical twin has the disease, his co-twin also has it.
b. Every time a fraternal twin has the disease, his co-twin also has it.
c. Identical twins reared together are both more likely to have the disease than are those reared apart. Both are also more likely to have the disease than are fraternal twins.
d. (none of these)

5. Which of the following research methods are useful in studying the nature-nurture issue?

a. cross-sectional studies
b. Family histories
c. Co-twin studies
d. Cross-cultural studies


Now take the UNIT TEST

Unit 7 Table of Contents

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August 24, 2002