Abstract
Light may be thrown on the aetiology of human abnormalities by studying the position in birth order occupied by affected individuals and the age of the mother at the time of their birth. There should be no shift from average population values in either of these respects in the case of conditions directly and solely due to an abnormal gene, though there may be such a shift in conditions due to chromosomal abnormalities, such as mongolism (Down's syndrome). If, however, the question of a chromosomal anomaly does not arise, then departure from normality in birth order and maternal age suggest that environmental factors are involved in the causation of the condition.

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