The Survival of Intelligence

The ordinary method of estimating the fertility of the stocks which produced any group by ascertaining the number of its siblings may be extended by the inclusion of cousins. An inquiry of this sort was conducted recently by means of a printed questionnaire presented to the entire membership of Clark University, 31 controls from the city of Worcester (mostly nurses in training) and about a dozen faculty members from Cornell University. The question (excluding explanatory matter) took the form: "How many grandchildren did each of your grandparents have? Count each child born alive, and include yourself." It was answered by a total of 216 persons, two of the returns being incomplete. The results take the form of four values for each individual, one for each of his grandparents. It is necessary to derive an index which shall combine these values in such a way as to show the number of individuals or equivalents of individuals in the propositus generation representing the four individuals of the grandparental generation. It may be observed at once that the propositus and his siblings represent the grandparents completely, being descended from them and from nobody else in their generation. Half-siblings and cousins, however, are descended in part from the grandparents in question, and in part from two other grandparents. The appropriate index, therefore, may be secured by adding to the number in the propositus' immediate family half the number of half-siblings and cousins; this, however, turns out to be the simple average of the four values, and this may, then, be referred to as the surviwal index. It is evident that, barring juvenile mortality, a survival index of 4.0 would be sufficient to maintain a stock; since, however, mortality by the age of 27 (which we may take as a reasonable reproductive age) is 25%70, we may take 5.3 as the appropriate maintenance value. The following are the percentages falling below 6.0 for the group studied, by subgroups: