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Wednesday, August 25, 2010

In Memoriam: James M. Tanner

The New York Times has announced the recent death of J.M. Tanner, an extraordinary figure in the field of child development. This was the man who established the scientific study of children’s physical growth and thus opened the door for further research and for practical applications of this information. It was Tanner’s work on rates of growth and development that made possible the study of changes in the age of puberty and that helped neonatologists discriminate between babies who were born prematurely and those who were simply very small for their stage of development. When I needed to evaluate the status of a poorly-nourished child a few weeks ago, I went straight to my well-used copy of Tanner’s book Foetus into man and looked at the appropriate growth chart, which showed proportions of children of a certain age at or below given weights. Indeed, almost every parent has seen his or her child’s height and weight marked on growth charts derived from Tanner’s work.

A part of Tanner’s contribution was simply a matter of meticulous measurement of large numbers of cases. He recorded heights, weights, and other details of growth for large numbers of children of known ages, and dealt with issues like the differences between measurement of height for infants (lying down) and for older children (standing up),with the problems these presented for accuracy. A particularly important question involved tracing the events of development before and during puberty; Tanner established a set of data based on photographs and measurements of changes in girls’ nipples and breasts, in pubic and axillary hair, and in boys’ penis and scrotum characteristics. This information allowed him to establish ways of predicting menarche and spermarche with some accuracy, an important step in deciding how to provide suitable sex education for older children as well as establishing standards for precocious puberty.

Of course, a set of data alone is not much help for understanding or decision-making. Tanner treated his collected data as the normal distribution they were, and was able to use related statistical concepts to help identify abnormal growth patterns that needed treatment. With a large set of data from children of various ages, Tanner could establish the statistical average heights and weights at specific ages by adding all the figures from an age group and dividing by the number of children. This helped to characterize particular children’s growth as being above or below average, of course, but it accomplished something still more important: a way to decide when growth was inside or outside a normal range, and thus whether a child did or did not need treatment. Most of the children in any group will have heights or weights that are above or below the statistical average, so that comparison is not a very useful one-- but knowing how far away from the average someone is can be extremely helpful when it comes to identifying problems.

Tanner did the statistical work that allows us to look at a growth chart and to decide how far a given child is from the average height or weight for his or her age. Like others making statistical comparisons, he usually considered a “normal range” within which children did not need help. To establish the normal range, he calculated the weight or height than which only 10% of an age group were smaller (the 10th percentile, the number below which 10% of the heights or weights fell), and the weight or height than which only 10% were larger (the 90th percentile, the number below which 90% of the heights or weights fell). Children whose measurements were in the top or bottom 10% groups were outside the normal range, and the other 80% of children were within the normal range. Children who were bigger or smaller than that normal range might be in need of treatment; for younger children, being unusually small was of greatest concern because the condition might be associated with other problems.

Establishing a normal range for children of different ages was especially important for care of premature babies. Tanner’s work included measurement of fetuses whose gestational age (time passed since conception) was known. Babies who were small at birth could then be classified as small, average, or large for their gestational ages, rather than all being considered simply premature even though some might be full-term births. This kind of classification made it possible to understand and treat the differing problems associated with different ages and relative sizes, rather than trying the same general treatment for all very small babies.

Tanner’s enormously useful and influential work seems to have been related to the maturationist descriptive research of Arnold Gesell, who in the 1930s provided detailed information about early changes in functions like grasping and handedness. The matters that Gesell studied were, of course, much less easy to measure and analyze statistically than height and weight were, and no one has been able to establish the same types of comparisons to a normal range that Tanner achieved with his studies of growth. However, Tanner’s work has set a standard for the study of development that may inspire investigation of more complex events, perhaps some day allowing for much earlier diagnosis of problems like autism.

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