Our interpretation of these coefficients relies on attributing the change in trends after 1924 to the introduction of iodized salt. If, however, high and low goiter states were trending differently before 1924, this would suggest that the difference in trends after 1924 may not be due to salt iodization. In order to test for the existence of differential pretrends, we add cohorts born in an even earlier period (1916–1919) and include a “pre-1920” indicator for cohorts born in this period, interacted with the goiter variable. Results are reported in table 3, where the pre-1920-by-goiter coefficient estimates the difference across the goiter distribution in cohort trends prior to the introduction of iodized salt. Across all specifications, these pre-1920 coefficients are not significantly different from 0. This alleviates concerns that states were experiencing different cohort trends—before 1924—systematically correlated with the goiter distribution. Moreover, the during-by-goiter and after-by-goiter coefficient estimates are almost identical to those in the previous table.

Table 3.
Pretrends in Labor and Income Outcomes
(1)(2)(3)(4)
1(Employed)1(Participated in the Labor Force)1(Worked at least 40 weeks)sinh$-1$(Income)
After $×$ Goiter Rate 0.00707*** 0.00680** 0.00880** 0.107***
(0.00248) (0.00284) (0.00370) (0.0288)
During $×$ Goiter Rate 0.00356 0.00323 0.00854*** 0.0274
(0.00231) (0.00233) (0.00269) (0.0259)
Pre-1920 $×$ Goiter Rate −0.00317 −0.00264 −0.00422 0.0107
(0.00210) (0.00183) (0.00370) (0.0253)
Observations 3,114,884 3,114,884 1,940,335 2,784,412
Mean of dependent variable 0.635 0.660 0.794 7.930
(1)(2)(3)(4)
1(Employed)1(Participated in the Labor Force)1(Worked at least 40 weeks)sinh$-1$(Income)
After $×$ Goiter Rate 0.00707*** 0.00680** 0.00880** 0.107***
(0.00248) (0.00284) (0.00370) (0.0288)
During $×$ Goiter Rate 0.00356 0.00323 0.00854*** 0.0274
(0.00231) (0.00233) (0.00269) (0.0259)
Pre-1920 $×$ Goiter Rate −0.00317 −0.00264 −0.00422 0.0107
(0.00210) (0.00183) (0.00370) (0.0253)
Observations 3,114,884 3,114,884 1,940,335 2,784,412
Mean of dependent variable 0.635 0.660 0.794 7.930

Standard errors, clustered by state of birth, in parentheses: ***p < 0.01, **p < 0.05, and *p < 0.1. “Goiter Rate” is the goiter rate in the individual's state of birth from Love and Davenport (1920), scaled by the difference between the 75th and 25th percentile of the goiter distribution (0.71). “Pre-1920” is a dummy equal to 1 for those born before 1920. “After” is a dummy equal to 1 for those born 1928 to 1931. “During” is a dummy equal to 1 for those born 1924 to 1927. These regressions use the 1950 to 1980 Censuses, restricting to individuals born from 1916 to 1931. All regressions include state of birth fixed effects; year of birth $×$ census year dummies; census division of birth $×$ birth year dummies; gender; race; and pre-1920, during, and after dummies interacted with average state latitude and 1920 state-level female and black proportions. “1(Worked at least 40 weeks)” is conditional on having worked in the past year. “sinh$-1$(Income)” takes the inverse hyperbolic sine of total income, including zeros for those not working.

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