Our study is built on a previous study incorporating more characteristics (called phenotypes) and ages. Understanding the conceptual idea is the most difficult part. Within-family genetic effects are calculated by comparing siblings, in our case twins. The genetic differences among siblings are almost random, coming purely from the random process of inheritance during meiosis and sometimes described as the “Mendelian lottery”. In contrast, differences between families are systematic: they reflect stable advantages or disadvantages that affect all siblings in a household.
By comparing polygenic score prediction within families to prediction in the population, we can estimate how much of genetic prediction reflects random inheritance versus broader family-level processes.
What we found
Our results showed that educational and cognitive outcomes are significantly influenced by between-family genetic effects. On average, about half of the population-level prediction from polygenic scores for these outcomes came from between-family differences.
In other words, genetic prediction of cognitive abilities and educational outcomes reflects both:
- random genetic differences between siblings, and
- systematic family-level advantages that are indexed by DNA.
We also found that while the overall predictive power of polygenic scores increases with age, the balance between within-family and between-family effects remains stable over development.
Prediction versus explanation
Since within-family differences are more “random” while between-family differences are more “systematic” operating through social and environmental processes, some researchers may argue that only within-family effects are “valid” genetic effects and that between-family effects are confounding. However, this interpretation can be misleading, as the most “random” genetic effects do not operate in a vacuum. Instead, genes always exert their influence through environmental context.
We argue for a more nuanced view. If our goal is to explain how a trait develops, separating within-family and between-family effects is useful. But if our goal is to predict outcomes, then combining these effects can be both valid and informative. After all, between-family influences, whether biological or not, matter profoundly for people’s lives.
Ironically, the context where within-family prediction would be relevant is embryo selection, a highly controversial application of polygenic scores, particularly for traits like intelligence and educational achievements.
For most real-world uses of polygenic scores, including research, education, and public understanding of genetics, between-family effects are part of the signal, not a nuisance to be ignored.
Public engagement