Hello,
I was just curious whether it was common for families with multiple children to have rather large gaps in their iq score distribution. I was just wondering if this was a common occurrence or not.

It is my understanding that sibling scores tend to be quite close. If I find a study which mentions the typical point spread among siblings, I will post it. Meanwhile, someone else may post it.


That said, a family may have children with different birth experiences, health issues, or genetic conditions which may greatly influence brain development.

A family could also undergo a tremendous change which may impact the family focus on education, amount of time, attention, and support available to encourage some of the siblings. For example, any of these life-altering events or combination of similar events: death of one or both parents, loss of support system such as grandparents, aunts, uncles, job loss, health crisis, incarceration and/or legal issues.

Some families may be immersed in a culture which emphasizes education for males but does not value or encourage intelligence in females.

IQ is often to said to be comprised of Fluid Intelligence (native ability for logic and reasoning) and Crystal Intelligence (what one has learned through education and/or experience).

Additionally, scores on different test instruments are not necessarily equivalent. This chart lends some insight: https://www.hoagiesgifted.org/highly_profoundly.htm

The general finding is that siblings typically post full scale scores within about 10 standard scores of each other on a deviation IQ. This doesn't mean, of course, that there won't be outliers. Most of my sibling group fall within 10 points of each other, but one person is about 50 points (estimated, since that person is well off of the standard norms) higher than the rest of us.

Another situation is if any of the siblings have significant intra-individual diversity, in which case the global measure may not fully capture their reasoning abilities. A related factor may be differences in their testability at the time of assessment (e.g., very young, with unremediated second exceptionalities, at different stages of majority culture language acquisition (if coming from a language minority background)).

Indigo also mentioned other external factors, such as ACEs (adverse childhood experiences), which may artificially lower some children's performance on cognitive measures more than other's.

So in answer to your question: Is it common? Not really. But is it significantly unusual? Not necessarily. It depends on what cutoff you use for "common" and "unusual". Not helpful, I know! But if you consider that much of the research on heritability of intelligence suggests that it is about 0.5, then it is a bit easier to see how there might be a decently-sized base rate of families with large ranges in the overall population.

Related to this topic, here is some recent research including IQ and nature-v-nurture:

1) link to article -
https://www.psypost.org/2022/05/psy...-south-korea-and-the-united-states-63091
Psychologists found a “striking” difference in intelligence after examining twins raised apart in South Korea and the United States
by Eric W. Dolan
May 7, 2022
PsyPost

2) pre-publication (Aug 2022) -
https://www.sciencedirect.com/science/article/abs/pii/S0191886922001477
Personality traits, mental abilities and other individual differences: Monozygotic female twins raised apart in South Korea and the United States
by Nancy L. Segal, Yoon-Mi Hur
Science Direct, Journal of Individual and Personality Differences