Register data suggest lower intelligence in men born the year after flu pandemic

Objective To test the hypothesis that prenatal exposure to the Hong Kong flu, an influenza pandemic that haunted Europe during winter 1969 to 1970, was associated with reduced intelligence in adulthood. Methods Data from the Medical Birth Register of Norway were linked with register data from the National Conscript Service. The sample comprised all registered boys born alive in single birth after 37 to 43 weeks' gestation during 1967 to 1973 (n = 205,634). Intelligence test scores, recorded at military conscription, were available for 182,913 individuals. Results The mean intelligence score increased from one birth year to another, except for a downturn in 1970. The birth year 1970 was inversely associated with intelligence score (−0.03 standard deviation [SD]; p < 0.001) after adjustments for birth characteristics, parental characteristics, and the trend of increasing scores over the 7 birth years. Analyses with the sample stratified by birth month showed that the inverse association between the birth year 1970 and intelligence score was significant only among men born in July (−0.04 SD; p = 0.049), August (−0.05 SD; p = 0.013), September (−0.09 SD; p < 0.001), and October (−0.06 SD; p = 0.008). Thus, the intelligence scores of the men born 6 to 9 months after the epidemic were lower than the mean values for the men born in the same months a few years before or after. Interpretation Early prenatal exposure to the Hong Kong flu may have interfered with fetal cerebral development and caused reduced intelligence in adulthood. Ann Neurol 2009;66:284–289