SEXUAL CHARACTERISTICS IN EARLY ADOLESCENCE

The average timing of pubertal events in European and North American girls and boys has been graphically depicted by Marshall and Tanner. Even within this relatively homogeneous group, there are wide age variations: in girls, for instance, the onset of breast budding varies from age 8 to 13 years, and menarche ranges from age 10 to 16.5 years. In boys, the acceleration of testicular growth may start anywhere from age 9.5 to 13.5. The penile growth spurt starts between age 10.5 and 14.5 and the penis reaches adult size between age 12.5 and 16.5. The development of pubic hair may continue into adulthood. In contrast to the age at which the pubertal events occur, the sequence of events is much less variable. However, the sequence is not identical for all boys or all girls; also the rate of passing through the whole sequence varies considerably between individuals, and some sexual characteristics may mature relatively faster than others. Both adolescent girls (especially between the ages of 11 and 14 years) and boys (particularly the age group from 13 to 16 years) show a tremendous variation in somatic developmental status, which is one of the important factors explaining the typical problems of adolescent self-image and behavior.

The age at onset of puberty varies because of many factors, genetic as well as environmental. There are differences even between Western European countries. For example, the current mean age at menarche is 12.5 years in Germany, 13.0 years in England, and 13.4 years in Switzerland (Bierich). Socioeconomic status also has a strong influence: menarche occurs several months earlier in girls of a higher social class than in girls of a lower social class. This has been demonstrated in as different geographic regions and racial groups as Danes or African Bantu (Burrell and others), Indians (Israel) or Rumanians (Stukovsky and others). Higher social class usually implies better living conditions, including nutrition, sleep, and exercise, and these may be the major factors accounting for class differences in rates of growth in childhood as well as in timing of the growth spurt and of menarche.

Increased growth in childhood and early onset of menarche may be tied together by the critical weight hypothesis according to which menarche occurs in females who have reached a “critical weight,” associated with a decline in metabolic rate and with achievement of a characteristic body composition. More recent studies have shown that menarche correlates more closely with body composition than with the critical weight (e.g., Frisch and others) which suggests that menarche requires a critical level of fat stored in the body. There are corresponding standards for predicting the minimum weight, at a given height which is necessary for menarche to occur. The theory is not yet generally accepted, however. The direction of the causal relation between body composition and menarche is under investigation; the validity of the findings and conclusions have been questioned on statistical and other grounds.

A particularly interesting phenomenon is the acceleration of puberty. In Western countries, the onset of puberty seems to have gradually dropped in age over the last 150 years. For example, the age of menarche has fallen from seventeen to thirteen years during this period. Acceleration of puberty has also been demonstrated in non-Caucasian populations, for instance, in China or in Japan. Most likely, this secular trend is due to the complex changes in nutrition, social conditions, and public health, brought about by the technological development of the modern industrialized society.

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