The interplay of genetic inheritance and environmental factors on intelligence quotient (I.Q.) is illuminated by studies of twins and siblings. The expected sequence of I.Q. correlation, from strongest to weakest, across various kinship and upbringing configurations is as follows:
- Identical twins raised together (C): This pairing exhibits the highest I.Q. correlation due to shared 100% of genes and congruent environmental influences.
- Identical twins raised apart (A): Despite differing environments, their shared 100% genetic makeup results in a strong I.Q. correlation, slightly less pronounced than their co-habituating counterparts.
- Fraternal twins raised together (B): Possessing approximately 50% shared genes and experiencing identical environments, fraternal twins show a notable I.Q. correlation, though it is lower than that of identical twins.
- Siblings raised together (E): Sharing around 50% of genes and comparable environments, siblings typically display a lower I.Q. correlation than twins, often attributed to greater age differentials and diverse developmental experiences.
- Siblings raised apart (D): While genetically similar to siblings raised together, divergent environmental influences lead to the weakest I.Q. correlation among all studied groups.
This hierarchy corresponds to the confirmed order: (C), (A), (B), (E), (D).