Relationship between twins and intelligence

The individual twins' IQ's are normally distributed, with the mean = , SD = The mean absolute difference between twins is (SD = ), the. There was a substantial correlation between spouses for IQ (r) and resemblance in identical twins was higher than in first-degree relatives. Keywords: genetics, high cognitive ability, twins, intelligence, talent .. variation within sex could contribute to the correlation between twins.

As I said, IQ almost certainly comes from a combination of genes and the environment. Both can explain why a child's IQ can be so different from his or her parents'.

The environment makes sense. If mom doesn't eat right or drinks too much alcohol while she is pregnant, the baby's brain won't develop to its full potential. Kids who grow up in a more stimulating environment will have higher IQs. But genetics can explain the wide range of possible IQs too because so many different genes are involved in developing and running a brain. It is possible, for example, to inherit all the higher IQ genes from each parent and leave the lower IQ ones behind.

Now the child will be brighter than the parent. And of course the opposite is possible as well resulting in a child with a lower IQ than expected.

Heritability of IQ

When a trait is passed on like this it is called polygenic many genes. Height is another example of such a trait. Click here to learn more about how these traits are passed on.

Next I want to focus a bit on how much of IQ comes from genes and how much from the environment. I think you'll see why most scientists think about half our intelligence is genetic and half is environmental. How in the world can a scientists tease out what is going on? If you've ever tried a Punnett square on something "simple" like eye color, you know what we're up against!

What scientists usually do is study twins. So you can tell a lot just by comparing each twin in a twin pair. This is just what scientists have done. In the results shown below, the closer the number is to 1. So if you and your twin score a 1. Here is a sampling of the data: Same person tested twice.

And that is just what we see.

Twins: Research into Nature and Nurture

Identical twins are definitely more likely to share the same IQ than are fraternal twins. So genes do play a role in IQ. One of my favorite examples to illustrate the effect of genes on intelligence is the Curie family.

In our society, the Nobel Prize is considered evidence of great achievement.

This has evidently happened for height: The value of heritability can change if the impact of environment or of genes in the population is substantially altered. The population in developing nations often has more diverse environments than in developed nations. Today, this can be prevented by following a modified diet, resulting in a lowered heritability.

A high heritability of a trait does not mean that environmental effects such as learning are not involved. Vocabulary size, for example, is very substantially heritable and highly correlated with general intelligence although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions and thus heritability is high.

Furthermore, there may be differences regarding the effects on the g-factor and on non-g factors, with g possibly being harder to affect and environmental interventions disproportionately affecting non-g factors.

However, that the opposite occurs is well documented.

Heritability measures in infancy are as low as 0. In contrast, studies of other populations estimate an average heritability of 0.

Environment and intelligence There are some family effects on the IQ of children, accounting for up to a quarter of the variance. However, adoption studies show that by adulthood adoptive siblings aren't more similar in IQ than strangers, [26] while adult full siblings show an IQ correlation of 0.

However, some studies of twins reared apart e. This shared family environment accounts for 0. By late adolescence it is quite low zero in some studies. There is a similar effect for several other psychological traits. These studies have not looked into the effects of extreme environments such as in abusive families.

Knowns and Unknowns " states that there is no doubt that normal child development requires a certain minimum level of responsible care. Severely deprived, neglectful, or abusive environments must have negative effects on a great many aspects of development, including intellectual aspects.

Heritability of IQ - Wikipedia

Beyond that minimum, however, the role of family experience is in serious dispute. There is no doubt that such variables as resources of the home and parents' use of language are correlated with children's IQ scores, but such correlations may be mediated by genetic as well as or instead of environmental factors. But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family?

Recent twin and adoption studies suggest that while the effect of the shared family environment is substantial in early childhood, it becomes quite small by late adolescence.

Understanding Genetics

These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children's lives make little long-term difference for the skills measured by intelligence tests. Non-shared family environment and environment outside the family[ edit ] Although parents treat their children differently, such differential treatment explains only a small amount of non-shared environmental influence. One suggestion is that children react differently to the same environment due to different genes.

More likely influences may be the impact of peers and other experiences outside the family. This factor may be one of the reasons why IQ score correlations between siblings decreases as they get older. Phenylketonuria is an example, [31] with publications demonstrating the capacity of phenylketonuria to produce a reduction of 10 IQ points on average. Knowns and Unknowns" also stated that: Thus it is not yet clear whether these studies apply to the population as a whole.

It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence. The children's IQs initially averaged 77, putting them near retardation. Most were abused or neglected as infants, then shunted from one foster home or institution to the next. Nine years later after adoption, when they were on average 14 years old, they retook the IQ tests, and all of them did better.

The amount they improved was directly related to the adopting family's socioeconomic status. The average IQ scores of youngsters placed in well-to-do homes climbed more than 20 points, to Adopting families tend to be more similar on, for example, socio-economic status than the general population, which suggests a possible underestimation of the role of the shared family environment in previous studies.

There was not any statistically significant interaction for non-verbal ability, but the heritability of verbal ability was found to be higher in low-SES and high-risk environments. They argued that heritability increases during childhood and adolescence, and even increases greatly between 16—20 years of age and adulthood, so one should be cautious drawing conclusions regarding the role of genetics from studies where the participants are not adults.

Furthermore, the studies typically did not examine if IQ gains due to adoption were on the general intelligence factor g. By contrast, the adopted children's g mainly depended on their biological parents SES, which implied that g is more difficult to environmentally change.

These studies showed that while the adoptive parents' IQ does correlate with adoptees' IQ in early life, when the adoptees reach adolescence the correlation has faded and disappeared.