Section HUMANS and ENVIRONMENT 99 World War, and later kept stable at 0.3–0.4, mostly due to relative stabilization of interfamily variation in fertility (family planning and birth control, predominantly by means of medical abortions). <...> Component due to differential prereproductive mortality, Im, has shown a 50-fold decrease during the 20th century as a result of social progress and improvements in medical care, and nowadays attained negligible values (<0.03). <...> It means that in these urban populations process of selection relaxation, with respect to both components – If and Im, has been almost completed. <...> An important predictable consequence of selection relaxation is an increase of population genetic load, which implies further development of genetic counseling. <...> Intensity of intergroup selection (based on interethnic variation in birth rates within each megalopolis), is many times lower than that of intragroup selection (interfamily variation), and also tends to decrease, presumably, as a result of demographic transition – changes of migrant’s reproductive behaviour in urban environment. <...> Judging by 1926 census data, at that time interethnic differentials in vital statistics in Moscow were much more signifi cant than now, and almost all ethnic groups demonstrated a positive natural increase. <...> Key words: Crow’s indices, fertility, mortality, selection relaxation, urban populations, interethnic differentials Contact information: Kurbatova Olga, e-mail: okurbat@list.ru. <...> COMPARATIVE ANALYSIS OF PHYSICAL DEVELOPMENT OF SCHOOLCHILDREN FROM DIFFERENT COUNTRIES Marfi na Olga Department of Anthropology and Ecology, Institute of History National Academy of Sciences of Belarus, Minsk, Belarus Comparative analysis of the main indices of physical development of children, teen-agers and youth at the age of 7–17 years from Belarus, Russia, Poland, Serbia, Bulgaria and Estonia was performed. <...> Through decades and from one generation to another, changes in physical development of children and youth, in different territorial and ethnic groups are important to study. <...> In Belarus, studies of children and youth physical development were conducted by the team of the Department of Anthropology and Ecology, Institute of History, National Academy of Sciences, from the beginning of 1980’s till the beginning of the 21st century. <...> It was shown that the positive secular trend was going on in the populations of towns and villages in the 1990’s, and that urbanization is an important factor infl uencing the age <...>