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intelligence

Although children's genes set forth their basic potential intelligence and the best adoptive family environment can never hope to transform a retarded child into a genius, there are some indications that a positive adoption experience can increase IQ (intelligence quotient) levels by as much as 15 points or more.

A study by Christiane Capron and Michel Duyme, French researchers at the University of Paris, looked at the socioeconomic status (SES) of birthparents and adoptive parents and found an apparent environmental impact of adoption on adopted children.

All the adopted children studied were adopted before the age of six months, and the average age of the adopted person at the time of the study was 14. (It would be fascinating to learn if the IQ differences remained constant when the adopted adults grew up and moved away from home. Some researchers speculate the effect of the adoptive parents' SES wanes as the adopted person ages.)

Considering only "high" and "low" SES of birthparents and adopters, with high SES individuals being physicians and executives and low SES being farmworkers and unskilled laborers, the researchers found clear-cut differences in the adopted children's scores.

"Children reared by high-SES parents have significantly higher IQs than those reared by low-SES parents," stated the researchers, referring to adoptive parents.

The highest IQs of adopted children were recorded when the SES of both the birthparents and the adoptive parents were high, and researchers found a mean IQ score of nearly 120. The lowest scores occurred when both the birthparents and adoptive parents were of low SES, and the average IQ was 92 points.

Probably the most likely actual scenario is the low SES birthparent and the high SES adoptive parent: In this case, the mean IQ score was about 104 points. (A person with an IQ score of about 100 is generally considered of "average" intelligence, and incremental increases of 10 points are significantly important.)

It appears the intelligence level of adopted children can be raised by nearly 16 points or more when the adoptive parents are of a high SES, despite the birthparents' SES. This difference can mean the difference between topping out educationally with a high school diploma or going on to obtain a college degree.

Capron and Duyme don't explain why or how higher adoptive SES homes apparently produce children with higher IQs. Said psychology professor Matt McGue in Nature, "It remains unclear whether the SES effect is related to access to quality education, the variety and complexity of intellectual stimulation in the home, the parents' press for scholastic achievement, or some other factor that differentiates between high- and low-SES homes."

According to McGue, other studies have correlated the adoptive mother's encouragement of the child to confidence and subsequent test performance.

Earlier studies found a stronger contribution from genetic factors, when some studies found the adoptive parents' contribution is greatest during the early years of childhood and decreases as the child grows older.

A longitudinal study reported in 1949 by Marie Skodak and Harold Skeel found a strong correlation between birthmothers and their adopted-away children. This study also indicated the genetic influence increased until the children became adolescents.

According to the findings of a study done by Joseph M. Horn on 300 adoptive families and the birthmothers of the adopted children, Horn concluded, "Adopted children resemble their biological mothers more than they resemble the adoptive parents who reared them from birth."

He found that children with higher-IQ birthmothers were more intelligent, saying, "Children from higher-IQ unwed mothers surpassed those from lower-IQ unwed mothers, even though the intellectual potential in their environments was comparable."

A study by Sandra Scarr and Richard A. Weinberg looked at the IQ levels of children adopted transracially and reported on their findings in Child Development.

According to the researchers, "Black and interracial children scored as well on IQ tests as adoptees in other studies. Individual differences among them, however, were more related to differences among their biological than adoptive parents."

The researchers also compared black children adopted transracially to black nonadopted children. The black adopted children scored above average in intelligence, with an average IQ of 110. The researchers also speculated that the transracially adopted children had an average IQ greater than their biological parents.

The researchers concluded that the high intelligence quotients of the adopted children indicated two key points: first, that race does not indicate for intelligence differences and, secondly, that "black and interracial children reared in the culture of the tests and the schools perform as well as other adopted children in similar families."

They also concluded that the children who were transracially adopted were responsive to the familial environment, and the adoptive parents provided "intellectual stimulation and exposure to the skills and knowledge sampled on IQ tests." (See also FETAL ALCOHOL SYNDROME; GENETIC PREDISPOSITIONS.)


Christiane Capron and Michel Duyme, "Assessment of Effects of Socioeconomic Status on IQ in a Full Cross-Fostering Study," Nature, August?17, 1989, 552-553.

Joseph M. Horn, "The Texas Adoption Project: Adopted Children and Their Intellectual Resemblance to Biological and Adoptive Parents," Child Development 54 (1983): 268-275.

Matt McGue, "Nature-Nurture and Intelligence," Nature, August?17, 1989, 507-508.

Robert Plomin and J. D. DeFries, "The Colorado Adoption Project," Child Development 54 (1983): 276-289.

Michael Rutter, Patrick Bolton, Richard Harrington, Ann Le Couteur, Hope Macdonald and Emily Simonoff, "Genetic Factors in Child Psychiatric Disorders-I. A Review of Research Strategies," Journal of Child Psychology and Psychiatry 31 (January 1990): 3-37.

Sandra Scarr and Richard A. Weinberg, "The Minnesota Adoption Studies: Genetic Differences and Malleability," Child Development 54 (1983): 260-267.

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