558 EVOLUTION, GENETICS, AND EUGENICS 



The average at death of these 669 persons was 64.7 years. The 

 child mortality (first 4 years of life) was 7.5 per cent of the total 

 mortality, 69 families showing no deaths of that kind. The group 

 is as a whole, therefore, long-lived. 



The problem was to measure the resemblance between brothers and 

 sisters in respect of longevity — to find whether knowledge of the age 

 at which one died would justify a prediction as to the age at death of 

 the others — or technically, it was to measure the fraternal correlation 

 of longevity. A zero coefficient here would show that there is no 

 association; that from the age at which one dies, nothing whatever 

 can be predicted as to the age at which the others will die. Since it is 

 known that heredity is a large factor in longevity, such a finding would 

 mean that all deaths were due to some accident which made the 

 inheritance of no account. 



In an ordinary population it has been found that the age at death 

 of brothers and sisters furnishes a coefficient of correlation of the order 

 of .3, which shows that heredity does determine the age at which one 

 shall die to considerable extent, but not absolutely.' 



The index of correlation^ between the lengths of life within the 

 fraternity in these 100 selected families, furnished a coefficient of 

 — .0163='= .0672, practically zero. In other words, if the age is known 

 at which a member of one of these families died, whether it be one 

 month or 100 years, nothing whatever can be predicted about the age 

 at which his brothers and sisters died. 



' Mary Beeton, and Karl Pearson, Biometrika, I, p. 60. The actual correlation 

 varies with the age and sex: the following are the results: 



COLLATERAL INHERITANCE 



Elder adult brother and younger adult brother 2290=*^ .0194 



Adult brother and adult brother 2853* .0196 



Minor brother and minor brother 1026=*= .0294 



Adult brother and minor brother — .0262=*= .0246 



Elder adult sister and younger adult sister 3464=*= -0183 



Adult sister and adult sister 3322=*= .0185 



Minor sister and minor sister 1 74^ =*= • 0307 



Adult sister and minor sister — .0260=*= .0291 



Adult brother and adult sister 2319=^= .0145 



Minor brother and minor sister 1435=*= -0251 



Adult brother and minor sister — .0062=1= .0349 



Adult sister and minor brother — .0274=*= .0238 



"The method used is the ingenious one devised by J. Arthur Harris 

 (Biometrika, IX, p. 461). The probable error is based on «= 100. 



