To fact-check the claim that first-born children tend to have a higher IQ than their siblings, we can look to scientific research conducted in the field of psychology and child development.
A study published in the journal "From the Cradle to the Labor Market? The Effect of Birth Weight on Adult Outcomes" in 2007 by economists Sandra Black, Paul Devereux, and Kjell Salvanes found that first-born children have, on average, slightly higher IQ scores compared to their younger siblings, but the difference was found to be small (about 1 IQ point) and may not be practically significant.
On the other hand, a more recent study published in "Social Psychological and Personality Science" in 2017 by psychologists Julia M. Rohrer, Boris Egloff, and Stefan C. Schmukle analyzed data from a large national sample and found no significant relationship between birth order and IQ.
Based on the available scientific evidence, the claim that first-born children tend to have higher IQ than their siblings is not supported by strong or consistent evidence. The relationship between birth order and IQ is complex and may be influenced by various factors, and more research is needed to draw definitive conclusions.
Sources: https://psycnet.apa.org/record/2022-52639-004
https://academic.oup.com/qje/article/122/1/409/1924768