During the years 1964 to 1971 a nationally representative sample of MZ and DZ twins and controls was followed through the Swedish compulsory school. The main purpose was to study physical and mental growth during puberty as well as heredity-environment influences on these growth processes.

After 20 years a follow-up has been made of this sample with the purpose of investigating heredity-environment influences on life situation and self reported health at mid-life in relation to background factors collected during adolescence. 43 pairs of MZ twins, 90 pairs of DZ same sex and opposite sex twins as well 322 controls agreed to participate. A questionnaire was sent out to this group dealing with their present life situation such as family structure, economy, education and occupation. Other areas of interest were self reported health and ability to cope with their present life situation. The disadvantages found for MZ female twins at adolescence seem to persist at mid-life and ratings of school adjustment were related to coping ability as grown ups. This first report presents comparisons between twins and controls as well as males and females. To estimate heredity influences intraclass correlations for the twin pairs are calculated and for coping ability such influences seem to be quite conspicuous. Sex specific factors seem to operate regarding coping ability and satisfaction in educational choice and level. Generally, the results indicate some advantages for the males regarding self reported life situation and health. Possible reasons for such sex differences are discussed. This study has been supported by grants from the Swedish Council for Social Research.

This study is based on 145 pairs of twins included in a longitudinal study of a cohort of singletons born in 1953 and living in the Stockholm metropolitan area in 1963. Of these 145 pairs, 28 were classified as MZ, 103 as DZ and 14 pairs could not be classified. School achievement and ability test results have been collected for this group as well as data for the boys from physical and mental examinations on enrollment to military service. The purpose of this study was to compare within-pair similarity for MZ and DZ twins from birth onwards. There seems to be a divergent trend over time for MZ twin pairs to become more similar and DZ twin pairs to become less similar in height growth, school achievement and verbal ability. This is in accordance with previous research in this field. In relating similarity within pairs to birthweight, we found a drop in within-pair similarity for low birthweight MZ and DZ twin pairs (both twins ≤2.500 g), but not for normal weight twin pairs. A discussion is initiated concerning methodological implications of these results.