One cause? Many causes?

Abstract The use of bimodality in the distribution curve of a sample as evidence bearing on the existence of a single main factor in the production of a characteristic is discussed. It is pointed out that bimodality (double peaking) can arise through a number of different mechanisms other than the operation of a single factor of overwhelming importance. Conversely one cannot deny the existence of such a major factor simply from the absence of bimodality. The conditions which regulate the separation of a mixed population into two peaks are discussed in some detail. The existence of two populations should be deduced from bimodality only with the greatest circumspection; the cleanness of the segregation is in practice rarely good. The use of arbitrary dividing lines is deplored because of the risk of creating fictitious problems.

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