Why Do We Believe What Science Says? 29 



what scientific knowledge is and how it can be used. Such 

 misunderstandings are very significantly involved in public 

 discussion of problems of great importance. The nature of 

 these difficulties becomes much clearer if we consider a 

 currently unsettled problem like the relationship between 

 lung cancer and smoking rather than a long-agreed-upon 

 matter like the infective cycle of yellow fever. 



At the present time the lung cancer problem stands 

 about as follows. The observed incidence of lung cancer 

 in Europe and the United States has risen very substantially 

 since 1920. The per capita consumption of cigarettes in 

 the United States has risen in the same period from 1.89 

 lbs. per year to 9.61 Ibs.^ (1 lb. = 339 cigarettes) Males 

 have the disease about 6 times as often as females.^ During 

 the 1920's and 1930's many more males than females 

 smoked cigarettes excessively and there is still a consider- 

 able excess of male over female smokers. Coincident with 

 the increase in female smoking, there has been a rise in 

 female lung cancer. Further, if we ask 1,000 male patients 

 with lung cancer how much they smoke we find that only 

 five have never smoked at all and that 250 smoke more 

 than 25 cigarettes a day. In a group of 1,000 men of the 

 same age, weight, and social status who do not have cancer, 

 45 have never smoked and only 134 smoke as much as 

 two packages per day.'* Finally, if we gather together as 

 many people as we can who have never smoked or smoked 

 very Httle and match them against a group of people of 

 similar age who have smoked a lot, we find that the death 

 rate from lung cancer is 13 per 100,000 and 127 per 

 100,000 — in the two groups respectively.'^ 



You would think oflniand that this evidence would be 

 enough to convince the pubhc and their representatives 

 that smoking cigarettes is a serious public health problem 

 and that something should perhaps be done to reduce the 



