In which of the following cases would you expect to find the greatest similarity in IQ?

Correct Answer: Between identical twins

Identical (monozygotic) twins share nearly 100% of their genetic material. As a result, they generally show the greatest similarity in IQ compared with fraternal twins, non-twin siblings, or parent-child pairs. Environmental factors also influence IQ, but genetic similarity is highest among identical twins.

Why the other options are incorrect:

• Between fraternal twins: Fraternal twins share about 50% of their genes, similar to ordinary siblings, so their IQ similarity is generally lower than that of identical twins.
• Between non-twin siblings: They also share about 50% of their genes and usually have less similar IQ scores than identical twins.
• Between parent and child: A parent and child share about 50% of their genes, but their IQ similarity is generally lower than that of identical twins.