indicating that males in birth cohort that experienced a higher exposure to the risk of being drafted in the army during the Vietnam War were more likely to remain longer in college. Graphically, the effect of the draft is illustrated in figure 7 in which the measure of the risk of induction is plotted together with males and females college graduation rates, computed from the sample. The highest level of the induction risk corresponds with the cohorts where the male college graduation rate is peaking, both in absolute terms and relative to the female college graduation rate. The instrument is in effect defined at the birth cohort and gender level. In addition to the effect of the exposure to the Vietnam draft, the instrument could thus pick birth-cohort-specific effects or secular trends in differences between genders. In order to avoid this, all regressions include dummies for each birth cohort, a dummy for male and a trend accounting for the secular changes in males/females ratios. The robust standard errors are also clustered by birth cohort and gender, as suggested by Moulton (1990).

The instrumental variable results are displayed in the lower part of table 4. In columns 1 and 2, no control for Vietnam veteran status are included. The results of this instrumental variable regression (positive and significant effect of college education on smoking, but not significant on smoking cessation) are different than the ordinary least square results (college education decreases significantly smoking prevalence and increases significantly smoking cessation). However, the risk of induction in the army, which is determined by the birth cohort, could also affect smoking directly and not only via education. In this case, the risk of induction would not be a valid instrument.

One potential channel through which the risk of induction can be directly correlated with smoking behavior is that the risk of induction is directly correlated with the probability of becoming a Vietnam veteran. The ordinary least square results reported in table 3 all show a strong positive effect of being a Vietnam veteran on smoking prevalence (but not on smoking cessation).<sup>25</sup> Without controlling for actually being a veteran, the instrument of the induction risk picks both the effect of attending college (decreases smoking) and the probability of being a veteran (increases smoking).

The National Health Interview Surveys, however, include the information on the actual veteran status of the individual, so the direct effect of being a Vietnam veteran on smoking behavior can be isolated. Columns 3-8 of table 4 therefore control for actual Vietnam veteran status. Columns 3 and 4 do not control for income. In column 3, the instrumental variable estimate of the effect of college education on smoking prevalence is negative and significant, and is somewhat larger than the ordinary least square estimate (– 0.06 versus –0.047). The significance is lower in column 4, when smoking cessation is analyzed. When controlling for family income, as in columns 5 and 6, the instrumental variable estimates are very close to the ordinary least square estimates and the actual mean

> 25 Bedard and Deschenes (2003) also show that World War II veterans are more likely to smoke.

14