Beginning in 1954, the author studied 12 pairs of monozygotic twins, aged 22 to 77 years, who had been "reared apart since early childhood." The clinical research extended over a two- to four-year interval for each twin pair. The methods included indepth serial medical and psychiatric interviews, EEGs, ECGs, chest films, Wechsler-Bellevue intelligence measurements, and personality profiles as interpreted from the Rorschach test. To assist in confirming zygosity, blood groups were documented.
The current publication is a revised edition of the original, published in 1965, with a follow-up of the twins 25 years after the initiation of the investigation.
The results are discussed in four categories: health in general, physical illnesses, level of intelligence and personality profile, and presence or absence of mental illness.
Among the similarities between the twins of each pair are EEGs, level of functioning intelligence, and presence or absence of migraine headache. However, personality profiles showed