Psychic vulnerability as a sequel to perinatal morbidity. A longitudinal twin study with special reference to psychiatric morbidity and inter-twin dependency.

In this follow-up study 497 twins from 335 twin deliveries were examined between the ages of 12-20 years to ascertain late effects of perinatal complications and mild neurological abnormalities on mental health, by means of questionnaires filled in by the parents and by the twins themselves. One third of the twins had not suffered from any perinatal complications, while one third had been small for gestational age, one third had been born prematurely and one third had suffered from respiratory disorders. Neonatal hypoglycaemia had been found in 22% and hyperbilirubinaemia in 7% of the twins. Perinatal complications had occurred in almost all those twins who later underwent inpatient psychiatric treatment. Most perinatal problems were reflected in the twin's dependency on his co-twin, showing in particular a cumulative effect when occurring simultaneously. Those twins who are currently more dependent learned to walk and speak later than the non-dependent ones, and their EEG background activity of ten years ago, at the age of 2-10 years, was more abnormal than that of the non-dependent ones, indicating brain organic, i.e. probably perceptual, factors enhancing the development of inter-twin dependency. The dependent twins were more submissive and had the most frequent feelings of inferiority. The neurological, psychological and family interactional factors that cause a twin to distrust himself and depend on his co-twin are also discussed in this study.