Can Two Siblings Have Very Different IQ Scores and Abilities Despite Having Similar Upbringing and Parents with About the Same IQs?

Anyone familiar with multiple siblings can easily observe the differences in intelligence and personality. Despite being raised in similar environments and having parents with comparable IQs, siblings often exhibit significant variations in their intellectual capabilities and personal traits.

Understanding the Average IQ Difference Among Siblings

On average, the IQ difference between siblings is around 12 to 13 points. This is slightly less than the average difference of 17 points observed between two people randomly selected from the general population. This suggests that even siblings, who share a similar upbringing and genetic background, can have notable variations in their intellectual aptitudes.

Role of Genetics in Intelligence

Intelligence is largely influenced by the DNA we inherit. While environmental factors, such as nutrition and educational opportunities, play a role, genetic factors tend to dominate. Each person receives 50% of their DNA from each parent, which is selected through a highly random process. As a result, bright parents are more likely to have bright children, while dull parents are more likely to have children with lower IQs.

Heritability of IQ

The heritability of IQ, which refers to the degree to which variation in IQ can be attributed to genetic factors, is remarkably high. For IQ, the heritability is around 85%. Even more striking is the heritability of g, the general factor of intelligence, which can be as high as 91%. Researchers commonly use the following notations to discuss heritability:

A Additive polygenic - refers to genetic influences C Shared environment - refers to environmental factors shared within the family E Nonshared environment - refers to environmental factors that differ between individuals

During early childhood, the genetic influence (A) is relatively low, around 40%, but it increases to 85% by adulthood. This is known as the Wilson effect, showing the increasing influence of genetic factors over time.

Impact of Shared Environment

Studies have shown that the family environment has a minimal impact on intelligence. Notably, results from the Minnesota Transracial Adoption Study highlight this point. Sandra Scarr's findings suggest that by adulthood, the IQ correlation between biologically related individuals is entirely genetic. This implies that while siblings may share a similar upbringing, the genetic makeup has a more substantial impact on their intellectual similarities and differences.

According to Jensen, phenotypic general intelligence (g) closely mirrors genetic g but shows little resemblance to shared environmental g, indicating that shared family environments have little influence on IQ. This is further supported by Jensen's statement that by adulthood, the genetic contribution to IQ becomes fully determinant.

Genetic Basis for Intelligence

The number of single nucleotide polymorphisms (SNPs) that determine intelligence can number in the tens of thousands. For reference, there are approximately 4-5 million SNPs in a single person's genome. Scientists have identified more than 600 million SNPs across various populations, many of which are involved in intelligence. These SNPs are randomly distributed, affecting the expression of intelligence in different individuals.

Given the significant heritability of intelligence, any attempts to boost intelligence through environmental interventions, manipulations, or other means have proven to be practically ineffective. We are, in essence, the product of our genes, with environmental factors potentially exerting only marginal influence on our intellectual capabilities.

Conclusion

In summary, while siblings may share a similar upbringing and parents with comparable IQs, genetic factors play a dominant role in determining their intellectual differences. Understanding the genetic and environmental influences on IQ can help us appreciate the complexity and variability of human intelligence.