Reviews the current theories concerning individual differences in cognitive functioning. While some argue that heredity places a genetic cap on intellectual development, others emphasize the dynamic interrelationship between cultural and environmental factors. Concludes that intelligence is more complex than the heredity advocates allow. (MJP)

A case study of a pair of extremely deprived twin boys focuses on their verbal communication. Talkativeness alone was not regarded as a measure of intelligence, but attention was paid to the less talkative twin's greater use of imaginative speech. Speculations are made on the evolutionary role of creative speech. (IS)