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A paper was read by Dr. Bradford Hill before the Royal Statistical Society
on 21st January, 1936, on the recent trend of mortality from phthisis at young adult
ages. He considered generally the various factors which have been suggested to
account for the observed changes, and particularly (1) the possibility of a decline
of immunisation in childhood as a result of the fall in the death.rate from tuberculosis
producing a lower level of infection in early life; (2) changes in the occupational.
environmental conditions of young adults; and (3) changes in the regional distribution
of young adults through the slackening of migration from the country districts to the
towns. He found evidence implicating the third of these factors only, but concludes :
" It certainly seems unlikely that change in migration is the only factor influencing
the trend of a form of mortality which is of complex causation."
In discussing the paper, Dr. Percy Stocks said: "The general conclusion seems
to be that, in producing the local differences pointed out, local social economic conditions
of life on the one hand, and the selection produced by migration to new
spheres of work on the other, are both important factors, but neither of these factors,
nor yet the larger scale of employment of young women in shops and offices, can
account for the retarded fall of phthisis mortality in the country as a whole which was
evident up to 1932."
In the larger sense, therefore, the problem remains unsolved.