The panel approach to estimating the effects of the minimum legal drinking age focuses on the changes in the drinking age that occurred in most states in the 1970s and 1980s. We begin by presenting graphical evidence in on the relationship between the drinking age and the incidence of fatal motor vehicle accidents. The data underlying the series in come from the Fatality Analysis Reporting System for 1975–1993 for the 39 states that lowered their drinking age during the 1970s and 1980s. In figure, we present the time series of deaths due to motor vehicle accidents among: 18–20 year-olds during nighttime (solid circles); 18–20 year-olds during daytime (dotted line with hollow squares); and 25–29 year-olds during nighttime (stars). The time series in the figure are centered on the month in which a state took its largest step towards raising its drinking age back to 21. The daytime/nighttime distinction is standard in the literature (for example, Ruhm, 1996
; Dee, 1999
) and is useful for understanding the effects of young adult alcohol consumption because the majority (67 percent) of fatal motor vehicle accidents occurring in the evening hours (defined here as between 8:00 p.m. and 5:59 a.m.) involve alcohol, while only about a quarter of fatal motor vehicle accidents occurring in the daytime hours involve alcohol.
Deaths due to Motor Vehicle Accidents Recentered around the Time Period in which the Minimum Legal Drinking Age Was Raised back to 21
We also plot the percent of 18–20 year-olds that can drink legally in the 39 states that experimented with a lower minimum legal drinking age. This line does not drop instantly from 100 to 0 percent because some states increased their drinking age from 18 to 19 and then from 19 to 21 a few years later, and other states allowed people who could drink legally when the drinking age was increased to continue drinking legally.
reveals that, in the seven years after the increase in the drinking age, there is a substantial reduction in deaths among 18–20 year-olds due to nighttime motor vehicle accidents and much smaller reductions in deaths of 18–20 year-olds due to daytime accidents and of 25–29 year-olds due to nighttime accidents. That the largest reduction in death rates occurs for the type of accident most likely to drop in response to an increase in the drinking age is consistent with the possibility that the increase in the drinking age reduced the motor vehicle fatality rates of 18–20 year-olds. However, the graphical evidence in favor of the hypothesis that increasing the drinking age reduced deaths is not fully compelling. First, the decline in deaths due to nighttime motor vehicle accidents among 18–20 year-olds is not as abrupt as the decline in the percent of this population that can drink legally. Second, as can be seen in the figure, the number of 18–20 year-olds that die in nighttime accidents was already declining before the drinking age was raised in most states. For this reason turn to a state-level panel data approach that allows us to adjust for trends and time-invariant differences across states and estimate the effect of the minimum legal drinking on mortality rates.
To obtain an estimate of the decline in mortality attributable to the drinking age, we implement a panel regression analysis of the following form:
) is the number of motor vehicle fatalities per 100,000 person-years for one of four age groups: 15–17 year-olds, 18–20 year-olds (the group directly affected by changes in the drinking age), 21–24 year-olds, and 25–29 year-olds in state (s
) in time period (t
). For each age group, we separate daytime and nighttime motor vehicle fatality rates. As noted above, any effects of the minimum legal drinking age on motor vehicle fatalities should be primarily on evening accidents because they are much more likely to involve alcohol. The regressions include a dummy variable for each state (θs
) to remove time-invariant differences between states and dummy variables for each year (μt
) to absorb any atypical year-to-year variation.2
In addition, the regression includes state-specific linear time trends (ψst
). The inclusion of state-specific dummies in combination with the state-specific time trends mean that the regression will return estimates of how raising the drinking age changes the level of motor vehicle mortality in a typical state, while adjusting for any state-specific trends in outcomes that preceded the change in the drinking age. This approach lets us compare people born in the same state just a few years apart who became eligible to drink legally at different ages. The variable MLDA
(an acronym derived from Minimum Legal Drinking Age) is the proportion of 18 to 20 year-olds that can legally drink beer in state s
in time t
, and the coefficient on this variable is our best estimate of the impact on mortality rates of lowering the drinking age from 21 to 18.3
The regressions are weighted by the age-specific state-year population, and the standard errors clustered on state are presented in brackets below the parameter estimates (Bertrand, Duflo, and Mullainathan, 2004
The estimates of the effect of the minimum legal drinking age on mortality for the subgroups described above are presented in and are consistent with a large body of previous research showing that the minimum legal drinking age has economically significant effects on the motor vehicle mortality rates of young adults (for example, Dee, 1999
; Lovenheim and Slemrod, 2010
; Wagenaar and Toomey, 2002
). Specifically, we find that going from a regime in which no 18–20 year-olds are legally allowed to drink to one in which all 18–20 year-olds are allowed to drink results in 4.74 more fatal motor vehicle accidents in the evening per 100,000 18–20 year-olds annually. Relative to the base death rate for this age and time of day, this is a 17 percent effect (4.74/28.1 = 0.17), and it is statistically significant. The associated point estimate for daytime fatalities (the majority of which do not involve alcohol) among 18–20 year-olds is much smaller, both in absolute terms and as a proportion of the daytime fatality rate, and it is not statistically significant. In addition, the changes in evening fatalities among 15–17 year-olds and 25–29 year-olds (whose behaviors should not be directly affected by the drinking age changes) are not statistically significant, though the 95 percent confidence intervals around the point estimates for these groups cannot rule out meaningfully large proportional effects relative to the low average death rates for individuals in these age groups. Overall, these patterns are consistent with a causal effect of easier alcohol access on motor vehicle fatalities among the 18–20 year-old young adults whose drinking behaviors were directly targeted by the laws. However, the rate of motor vehicle fatalities in the evening for 21–24 year-olds also changes when the minimum legal drinking age changes. While the proportional effect size for 21–24 year-olds (2.61/23.2 = 0.1125, or about 11 percent) is substantially smaller than for 18–20 year-olds (17 percent), this approach does not have sufficient statistical power to reject that the two estimates are equal. The apparent effect of the minimum legal drinking age on fatalities among 21–24 year-olds could reflect the effects of other unobserved anti-drunk driving campaigns that were correlated with drinking-age changes and targeted at young adults, or it may reflect spillovers, as members of these two groups are likely to socialize.
Panel Estimates of the Effect of the Minimum Legal Drinking Age on Motor Vehicle Fatalities (deaths per 100,000)
In , we present estimates of the effects of the minimum legal drinking age on a more comprehensive set of causes of death. The mortality rates for this part of the analysis are estimated from the National Vital Statistics death certificate records. Since these records are a census of deaths and include substantial detail on the cause of death, it is possible to examine causes of death other than motor vehicle accidents. We present estimates of the effects of the minimum legal drinking age on all-cause mortality in using the same fixed-effects specification as in . Specifically, the dependent variable in each regression in the bold row of is the death rate of 18–20 year-olds per 100,000 person-years estimated from the death certificate records. All models in include state fixed effects, year fixed effects, and linear state-specific time trends. To increase the precision of the estimates, the regression are weighted by the size of the relevant population in that state and time period.
Panel Estimates of the Effect of the Minimum Legal Drinking Age on Mortality Rates (deaths per 100,000)
The first estimate for all-cause mortality in suggests that when all 18–20 year-olds are allowed to drink, there are 7.8 more deaths of 18–20 year-olds per 100,000 person-years (on a base of 113 deaths) than when no 18–20 year-olds are allowed to drink. This estimate is not statistically significant at conventional levels. Though the table reveals no evidence of a statistically significant increase in deaths due to internal causes (like cancer), it does reveal statistically significant increases in deaths due to motor vehicle accidents (4.15 more deaths on a base of 45.5 deaths, or a 4.15/45.5 = 0.091, or a 9.1 percent effect). This does not exactly match the estimate from because the Vital Statistics records do not include the time of day when the accident occurred, so we are unable to split the rates based on the time of the accident as we did with the earlier data.4
also shows that increasing the share of young adults legal to drink leads to a statistically significant 10 percent increase in suicides (1.29/12.8 = 0.10), which is consistent with work by Birckmayer and Hemenway (1999)
and Carpenter (2004b)
. There is no evidence of statistically significant effects on the other causes of death for 18–20 year-olds. The lack of a discernable impact on deaths directly due to alcohol is surprising, though in this period deaths due to alcohol overdoses appear to have been significantly undercounted (Hanzlick, 1988
In the remainder of , we present estimates of the relationship between the proportion of 18–20 year-olds that can drink legally and the mortality rates of three age groups: 15–17, 21–24, and 25–29 year-olds. Since the proportion of 18–20 year-olds that can drink should not directly affect these groups (except possibly through spillovers), these groups should experience at most modest increases in mortality rates. As can be seen in the table, with the exception of 21–24 year-olds there is no evidence of statistically significant changes in the mortality rates of the three age groups surrounding 18–20 year-olds. This suggests that the changes in mortality rates of 18–20 year-olds are probably not being driven by safety initiatives that may have been implemented at the same time the drinking age was increased as these would have affected the other age groups also. Overall, the patterns in and suggest that easing access to alcohol increases the overall death rate of 18–20 year-olds due to increases in two of the leading causes of death for this age group: motor vehicle accidents and suicides.