A 2017 study found that the introduction of iodized salt in 1924 raised the IQ for the one-quarter of the population most deficient in iodine. These findings "can explain roughly one decade’s worth of the upward trend in IQ in the United States (the Flynn effect)". The study also found "a large increase in thyroid-related deaths following the countrywide adoption of iodized salt, which affected mostly older individuals in localities with high prevalence of iodine deficiency". A 2013 study found a gradual increase in average intelligence of 1 standard deviation, 15 points in iodine-deficient areas and 3.5 points nationally.