In 1911 the census asked about deaf-mutism. This question was also asked in the next two censuses of 1921 and 1933. Deaf-mutism was found to be very high among 10- to 14-year-olds, with the same pattern existing in the 1921 census among 20- to 24-year-olds. The statisticians report on the 1921 census noted that it was "a reasonable assumption therefore that the abnormal number of deaf-mutes . . . was the result of the extensive epidemic of infectious diseases which occurred soon after many in those age groups were born". Rubella was not known to be a possible contributor. During World War II, the ophthalmologist Norman McAllister Gregg began to investigate the connection between birth defects and the infection of mothers early in their pregnancy. In 1951, prompted by Gregg's work, Australian statistician Oliver Lancaster examined the census figures of 1911, 1921 and 1933. He found a peak in the level of deaf-mutism in the age cohort born in 1898 and 1899 and that this matched with a known outbreak of rubella in those years. "This was the first time in the world that the link between rubella and congenital problems with unborn children was firmly established."