Human Intelligence Twin Studies and the Acquisition

Human Intelligence

Twin Studies and the Acquisition of Human Intelligence

The question of nature vs. nurture has been a topic of conversation, a hotly debated issue and reason for researchers to gather copious amounts of material for thousands of years. Philosophers discussed whether a child was mainly constructed of inborn (nature) or learned/observed traits (nurture) before Alexander the Great had conquered anything. Nature refers what is commonly called genetics today; nurture, conversely, is what an individual picks up from the environment. Many have been in one camp or another, but only recently have scientists had the ability to truly assess which is more correct.

One facet of this study, that of intelligence, may be the single greatest issue of discussion among scientists and lay persons. Intelligence as nature has taken a beating in the public arena due to such publications as "The Bell Curve." Many did not appreciate the findings, especially along racial lines in the book, so it was dismissed. Scientists have also shown many areas where the authors used faulty evidence. But, this essay is concerned with what nature vs. nurture truly is, how the study relates to intelligence, and an evaluation of the competing evidence in the argument.

Some terms must first be defined prior for this investigation to begin. Much of the evidence examined relates to twin studies performed with what are typically referred to as identical and fraternal twins. In these scientific studies identical twins are termed monozygotic (MZ) or coming from a single egg, and di-zygotic (DZ) or coming from different eggs (Mackintosh, 1998, 71). The identical twins will be referred to as MZ and the fraternal twins as DZ. Another term, common in many studies, is correlation. Eric Gander (2003, 83) defines this term as "the degree of similarity between two variables."

Psychology, generally being considered the study of behavior rather than specific physiology of the brain (although that does have some small part to do with psychology), wishes to understand whether an individual inherits mental traits from their parents before they are born or after. The question of genetics vs. The environment is paramount in understanding many of the different theoretical wanderings of different researchers such as the noted behaviorist James Watson. Many of the scientists have discussed the role of intelligence in the contrast of nature vs. nurture.

The genetics argument is that a child will gain a large amount of their intelligence makeup from heritability. This means that intelligence will vary little when compare to the intelligence of one's parents. Nurture, on the other hand, states that these traits of intelligence are largely gained after one is born. The environment in which ones lives and gains' understanding has more of an effect than does to whom the individual was born. Intelligence is at the core of the nature vs. nurture debate because it is a trait which people can anecdotally being passed from one person to another (such as eye color, height or weight). But, studies with both DZ and MZ twins who have been raised with their biological parents and raised apart have shown a light on which side of the argument is more true.

First, a not on intelligence studies in general. There has been some bias associated with the studies with regard to twins, especially thos who have been adopted and separated at birth. "The nonrandom placement of adopted? out twins further assures a high similarity between rearing families and biological ones. Again, a marked bias is present that leaves suspect any unqualified generalizations from the data"

(Farber, 1981, 18). This does not mean that the studies should e discounted, only that they should be carefully scrutinized before one relies on the results. But, this could be said about many studies.

To say the least, the research is not conclusive. It ranges from "low heritability indicates that differences are more related to the differences in their environments"

(Farber, 1981, 169) to "general intelligence is highly heritable,…its upper limit cannot be measurably increased through education" (Gander, 2003, 46). There is even the wishy-washy view that "there is increasing acceptance of the evidence for substantial genetic influence on many behavioral traits, but the same research also provides the best available evidence for the importance of environmental influence and important clues about how the environment works" (Plomin and Asbury, 2005). Basically, scientists cannot agree. So, the person who wishes to find out the truth of the matter has to find out what the incontrovertible evidence says and go with that. This evidence is mainly found in the many twin studies that have been conducted.

Mackintosh (1998) found that intelligence between identical twins is correlated at.86; between fraternal twins the correlation is .60; siblings .47; and, between unrelated children the correlation is .34 (71). This is backed up in a study which says that "an IQ study of 159 pairs of identical twins raised apart revealed a correlation of 75% while identical twins reared together had a higher correlation of 86%" (Schein and Bernstein, 2008, 56). While the second study does reveal a difference between MZ twins raised together and apart, the correlational difference between MZ twins and DZ twins continues to be significant (.75 to .60 correlation).

However, there is significant research that says, the above correlations notwithstanding, the environment plays a significant role also. Mackintosh (1998) says "MZ twins are more likely to dress alike, play together, share the same teacher, sleep in the same room, and be treated alike by others than are DZ twins" (72), and "If all MZ twins share more experiences in common than all DZ twins, then this difference between the two groups of twins might still be responsible for the difference between them in similarity of IQ" (73). He is saying that correlational differences could actually be attributed to environment, even though that is obviously not the only reason why there is such a high correlation. Collins, et al., (2003) states;