Intelligence is mental activity directed towards purposive adaptation to, and selection and shaping of, real-world environments relevant to one’s life. There tends to be a gap between definitions of intelligence and tests of intelligence. The key question is, is measured intelligence related more to hereditary or social and cultural influences. The most common measure of intelligence is the intelligence quotient or IQ.

The most popular method of assessing the relative importance of hereditary and environment in determining individual differences in intelligence is to conduct a twin study. There are two different types of twins monozygotic (MZ) and dizygotic (DZ). Monozygots have an identical genotype from one egg whereas dizygots come from two separate eggs. If heredity is very important then we would expect MZ twins to be considerably similar than tic twins. But if environmental factors are all important, then MZ twins should be no more alike then DZ twins. Bouchard and McGue (81) reviewed 111 studies on twins. They reported that the mean correlation for identical twins or MZ’s was +86 which is quite high, and +0.60 for fra

ternal twins or DZ’s. These findings would lead you to believe that genetic factors play an important role in determining intelligence, although they may do for twins it is not so true for relations. For example, siblings reared together had a correlation of +0.47. The fact that MZ twins were much more similar in intelligence than DZ twins suggests that heredity is of major significance in determining intelligence. However, that is on the assumption that the degree of environmental similarity experienced by MZ twins is the same as that to DZ twins. Evidence has shown that identical twins have similar IQ’s because due to the similar treatment they received such as, parental treatment and playing together.

What is needed to compare several different aspects of the environment in terms of the effects they have on children’s level of intelligence. A suitable measure for comparing several different aspects of the environment in terms of the effect they have on children's level of intelligence is the HOME inventory. The HOME inventory measures six environmental categories. These are emotional and verbal responsitivity or parents, avoidance of restriction and punishment, organisation of physical and temporal environment, provision of appropriate play material, parental involvement with child and opportunities for variety in daily stimulation. Gottfried (84) addressed the issues of which these six aspects have the greatest effect on children's IQ. Findings shows that play materials, parental involvement and variety of daily stimulation predicted children’s subsequent IQ better than did any of the three other aspects. The findings of Gottfried were correlational and can’t show what actually increases children’s IQ but it does show

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