This claim is true, according to multiple research studies, it is more likely for the first born child to have a slightly higher IQ than the second. Based on an article by Phys.org, “birth order does matter when it comes to IQ.” However, siblings’ IQ differences are determined more by parental strategies than by human biology. According to the article, much of a child’s enrichment comes down to the way they were raised, and since it is typical for the first born child to receive “undivided attention from the parents,” they are more likely to display higher levels of intelligence.
A piece from the New York Times further supports this claim, stating that the “average difference in IQ was three points higher in the eldest child,” based on a large-scale study. Additionally, the Illinois News Bureau did a study on 377,000 high school students, and found that firstborns did in fact have a higher IQ than their younger siblings. While these differences are small, they do appear to be consistent throughout the majority of research done on first and second born children.
However, some articles stated that the documented differences were insignificant, and that birth order “had no meaningful effect on personality or IQ.” Therefore, while the initial claim is true, the significance of the claim seems to be undermined by other factors, such as upbringing. Since early development is so significant for humans, whatever care they receive as children, or lack-there-of, will give them certain qualities.
The amount of mental stimulation firstborns are given appears to correlate with their intelligence, but that’s not to say a younger sibling couldn’t have a higher intelligence than their older sibling. Overall, the numbers provided from each study are simply averages.
Sources:
https://phys.org/news/2015-10-first-borns-higher-iq-sibling-bonds.html
https://www.nytimes.com/2007/06/22/science/22sibling.html
https://news.illinois.edu/massive-study-birth-order-has-no-meaningful-effect-on-personality-or-iq/
