The Hong Kong flu pandemic was responsible for more than 700,000 deaths worldwide in the late 1960s, with major disease outbreaks in Europe in the winter of 1969-1970.
A number of studies have been conducted to determine if prenatal exposure to the influenza virus may result in mental disorders that affect a small portion of the population, but no studies have explored the possible effects of prenatal exposure on the mean intelligence in the general population. A new study found that early prenatal exposure to the Hong Kong flu may have interfered with fetal cerebral development and caused reduced intelligence in adulthood. The study is published in Annals of Neurology .
The study involved records of more than 180,000 men born between 1967 and 1973 who served in the military. Military service is compulsory for young men in Norway, who are evaluated medically and psychologically before they enter the service. The intelligence test data used in the study consisted of a composite score from arithmetic, word similarity and figures tests similar to those commonly used in intelligence tests.
The results showed that the mean intelligence score increased in every birth year from 1967 to 1973, except for a downturn in 1970. The intelligence scores of men born in July through October of that year, six to nine months after the main outbreak of the Hong Kong flu in Norway, were lower than the mean values for those born in the same months during the preceding and following years. The mean intelligence score of men born during these months was also lower than the mean score of men born in any other month in 1970, and this trend was not seen in the other years. As the flu outbreak occurred during the winter months, this suggests that exposure during the first three to four months of pregnancy seems to have had the strongest impact on intelligence scores.