Eugenics Laboratory Lectures number 4: Elderton, Ethel M. 1909. On the Marriage of First Cousins (London: Dulau and Co.). 39 pp. (Cain suspects the 1909 edition of this volume was never printed formally in the series. The 1911 “second edition” is available.)
Summary
Authored by Ethel M. Elderton, this lecture statistically investigates consanguineous marriage, primarily focusing on the consequences of cousin marriage. The study relies on extensive family records and specialized pedigrees, notably for conditions like albinism.
The lecture first quantifies the hereditary resemblance between first cousins, finding an average value of 0.265 for various physical and psychological traits (excluding eye colour), a result roughly equal to the resemblance between grandparents and offspring. This value supports the general theory of heredity that resemblance decreases in geometrical progression in more distant relatives.
Elderton addresses the common belief that cousin marriages lead to lower fertility (“want of children”). Using Sir George Darwin’s earlier data and later albinotic pedigrees, she demonstrates that cousin marriages generally do not result in a diminution of fertility compared to non-related marriages.
However, the major finding is the amplification of hereditary defects. When a recessive hereditary abnormality (like albinism, ichthyosis, or deaf-mutism) is present in the stock, a consanguineous marriage acts as a severe danger. For instance, if both parents are normal but related, a cousin marriage markedly increases the number and intensity of affected children. If one parent is already affected, a consanguineous marriage can more than double the percentage of affected offspring (e.g., 32% versus 14% for albinism). Elderton emphasizes that this conclusion, proven for these three defects, holds “in all probability for a far larger range of pathological states”.
Eugenics Laboratory Lectures number 4
Alternative:
Elderton, Ethel M. 1909. On the Marriage of First Cousins (not yet located).