This section illustrates the use of longitudinal data in the study of growth and puberty, health behaviors, and self-rated health. Research results presented in Part II are based on JYLS data (Chapters 4 & 6) and FinnTwin12 and FinnTwin16 data (Chapters 4, 5, 6, 7). For the twin data, information from the parents is also used.

Chapter 4 by Kinnunen, Pietiläinen, and Rissanen examines the determinants of growth and the critical periods in the development of obesity from birth through childhood and adolescence to adulthood. In twin families, size of twins at birth is tracked to late adolescence and early adulthood but the tracking of height was more substantial than the tracking of relative weight. Length at birth and parents' height were the main predictors of final height and birth weight, whereas parents' body mass index (BMI) and mother's smoking during pregnancy were the most significant determinants of early adult BMI. In the JYLS sample, childhood BMI was a better predictor of adult obesity than size at birth. Young adults, especially the males, were at high risk of gaining weight in both study samples.

Chapter 5 by Dick and Mustanski documents the major contribution of genes to variation and familial aggregation of puberty assessed either as age at menarche or using measures such as the Pubertal Developmental Scale. However, several environmental factors importantly influence individual timing, and one is father absence; girls whose fathers are absent from the home experience puberty at an earlier age in our twin data.