The Editor: Nature or Nurture? 



233 



good reason to believe that its appear- 

 ance in any individual is largely due to 

 his inheritance of what physicians call 

 the tubercular diathesis — that is, a 

 weakness of the constitution predis- 

 posing to tuberculosis. As compared 

 with this factor, all factors of infection 

 are of relatively small importance. 

 Under modern city conditions, it is 

 almost certain that one who leads a 

 moderately active life will be exposed to 

 infection from tuberculosis every day. 

 Whether he succumbs or resists will 

 depend on his heredity. 



But, it is argued, at any rate bad 

 housing and unsanitary conditions of 

 life will make infection more easy and 

 lower the resistance of the individual. 

 Perhaps such conditions may make 

 infection more easy, but that is of 

 little importance considering how easy 

 they are for each of us — for the popula- 

 tion as a whole. The question remains, 

 will not bad housing cause a greater 

 liability to fatal phthisis? Will not 

 destitution and its attendant conditions 

 increase the probability that a given 

 individual will succumb to the white 

 plague ? 



CAUSES OF DEATH FROM PHTHISIS 



Most physicians think this to be the 

 case, but they have not taken the pains 

 to find out, by the exact methods of 

 modern science. We are accustomed 

 to take their word on the subject, 

 because we think they ought to know. 

 Dr. Knopf of New York, one of the 

 country's authorities on tuberculosis, 

 recognizes the importance of the hered- 

 ity factor, but says that after this, the 

 most important predisposing conditions 

 are of the nature of unsanitary schools, 

 unsanitary tenements, unsanitary fac- 

 tories and workshops. This may be 

 very true; these conditions may follow 

 after heredity in importance — but how 

 near do they follow? That is a matter 

 that is capable of fairly accurate 

 measurement, and we ought to have 

 figures, not generalities. 



Taking the case of destitution, which 

 includes, necessarily, most of the other 

 evils specified, Pearson measured the 

 correlation with liability to phthisis and 

 found it to be .02. The correlation for 



tuberculosis between parent and child, 

 on the other hand, was found from 

 several investigations to be about .50 — 

 just what we should expect, for that is 

 the correlation in general for physical 

 and mental characters in heredity. It 

 is also the correlation for the inheritance 

 of such pathological characters as insan- 

 ity and congenital deafness, where 

 certainly infection of child by parent 

 could -not be suspected. 



Nevertheless, many thought that the 

 high correlation between parent and 

 child, in the case of tuberculosis, must 

 be due to infection. The family rela- 

 tions are so intimate, they said, that it 

 is folly to overlook this factor in the 

 spread of the disease. 



Very well, Pearson replied, if the 

 relations between parent and child are 

 so intimate that they lead to infection, 

 they are certainly not less intimate 

 between husband and wife, and there 

 ought to be just as much infection in this 

 relationship as in the former. The 

 correlation was measured in thousands 

 of cases and was found to lie around .25, 

 being lowest in the poorer classes and 

 highest in the well-to-do classes. 



At first glance this looks like a damag- 

 ing correlation — it looks as if there must 

 be a considerable amount of tubercular 

 infection between husband and wife. 

 But when we find that the resemblance 

 between husband and wife in the 

 matter of insanity is also .25, we must 

 pause. Certainly it will hardly be 

 argued that one of the partners infects 

 the other with this disability ! 



ASSORTATIVE MATING 



As a fact, this correlation of say .25 

 between husband and wife, for tuber- 

 ciilosis, probably means very little for 

 infection. What it does mean is that 

 like tends to mate with like — in the 

 more prosperous classes, at least, where 

 there is a considerable range of choice. 

 It means assortative marriage, sexual 

 selection. This coefficient of resem- 

 blance between husband and wife in 

 regard to phthisis is about the same as 

 the correlation of resemblance between 

 husband and wife for eye color, stature, 

 longevity, general health, truthfulness, 

 tone of voice, and many other charac- 



