摘要：Objectives. We estimated the effects on assault rates of lowering the minimum alcohol purchasing age in New Zealand from 20 to 18 years. We hypothesized that the law change would increase assaults among young people aged 18 to 19 years (the target group) and those aged 15 to 17 years via illegal sales or alcohol supplied by older friends or family members. Methods. Using Poisson regression, we examined weekend assaults resulting in hospitalization from 1995 to 2011. Outcomes were assessed separately by gender among young people aged 15 to 17 years and those aged 18 to 19 years, with those aged 20 and 21 years included as a control group. Results. Relative to young men aged 20 to 21 years, assaults increased significantly among young men aged 18 to 19 years between 1995 and 1999 (the period before the law change), as well as the postchange periods 2003 to 2007 (incidence rate ratio [IRR] = 1.21; 95% confidence interval [CI] = 1.05, 1.39) and 2008 to 2011 (IRR = 1.20; 95% CI = 1.05, 1.37). Among boys aged 15 to 17 years, assaults increased during the postchange periods 1999 to 2003 (IRR = 1.28; 95% CI = 1.10, 1.49) and 2004 to 2007 (IRR = 1.25; 95% CI = 1.08, 1.45). There were no statistically significant effects among girls and young women. Conclusions. Lowering the minimum alcohol purchasing age increased weekend assaults resulting in hospitalization among young males 15 to 19 years of age. Hazardous consumption of alcohol is a leading contributor to the global burden of disease, causing 3.8% of all deaths and 4.6% of disability-adjusted life-years (DALYs). 1 In New Zealand, it causes 5.4% of deaths and 6.5% of DALYs. 2 The burden is borne disproportionately by men (8.8% of DALYs vs 4.3% of DALYs among women), the young (mortality rates peak among those aged 15–29 years), and Māori (the indigenous people of New Zealand, whose alcohol-related mortality rate is 2.5 times that of the non-Māori population). 2 The economic burden of hazardous drinking is estimated to be 4% of the country’s gross domestic product, 3 placing it among the most costly modifiable risk factors in New Zealand. There are various evidence-based strategies for reducing alcohol-related harm, prominent among them restricting the availability of alcohol to young people via a minimum drinking or purchasing age. 4 In 1999, contrary to this evidence, the New Zealand Parliament lowered the minimum alcohol purchasing age from 20 to 18 years. Four studies of the short-term effects of that law change on young people have been published in the scientific literature; they showed increases in emergency department admissions for intoxication, 5 increases in disorder offenses and drunk driving, 6 and higher traffic crash injury rates than would have been expected in the absence of the change. 7,8 Injuries stemming from assault have rarely been studied as an outcome of a change in the minimum alcohol consumption or purchasing age. 9,10 Etiologic fractions estimated in the Global Burden of Disease project suggest that between one third and one half of assaults resulting in hospitalizations are attributable to alcohol consumption on the part of either the assailant or the victim. 2 Efforts to evaluate the short-term effects of New Zealand’s law change on assaults have been thwarted by data limitations and insufficient statistical power. 11 Data limitations have included the lack of a routinely used indicator of alcohol involvement in hospital records. 12 Analyzing all assaults—irrespective of alcohol involvement—is likely to increase the risk of type II error because many are not alcohol related and their inclusion adds noise to the data. Lack of knowledge about the impact of this law on assault rates is a shortcoming with respect to public health policy because, in contrast to the situation with traffic crash injuries, there are no evidence-based targeted interventions (e.g., roadside breath testing or sobriety checkpoints) to ameliorate the effects of the increased availability of alcohol brought about by lowering the minimum drinking or purchasing age. Our conceptual model of the basic mechanism of the law change is shown in Figure 1 . Reducing the minimum purchasing age from 20 to 18 years increased legal access to alcohol among young people aged 18 to 19 years. It is also likely to have increased informal access through those aged 18 to 19 years supplying alcohol to their friends and siblings younger than 18 years, which was legal in the context of a “private party” under New Zealand law until December 2013. Research conducted in 2000 showed that underage purchases of alcohol were relatively common. 13 Although it is likely that prescriptive norms concerning alcohol consumption among those aged 18 to 19 years became more permissive after the law change, data are lacking on this presumption. Open in a separate window FIGURE 1— Conceptual model of the effects of lowering the minimum alcohol purchasing age on assault rates. All of these factors are likely to increase alcohol consumption and the exposure of young people aged 18 to 19 years to licensed premises where they encounter other young people impaired by alcohol. Greater alcohol consumption and increased exposure to other alcohol-impaired people in licensed premises would be expected to independently and multiplicatively increase the probability of involvement in an assault. With the additional person-years of exposure accumulating since the 1999 law change, we sought to evaluate the effects on weekend assaults (Friday through Sunday), incidents in which there is a high probability that alcohol consumption is a contributing factor, thus reducing the degree of type II error. There is evidence from other countries that assaults are much more common during weekends than earlier in the week 14,15 and that the proportion of assaults attributable to alcohol is also greater on weekends. 15 Accordingly, we tested a pair of hypotheses using young people aged 20 to 21 years as a control age group. First, we predicted that weekend assaults would increase among those aged 18 to 19 years, the age group whose legal access to alcohol increased with the law change. Second, we predicted that assaults would increase among those aged 15 to 17 years, a group with an already high prevalence of hazardous drinking 16 in whom informal access to alcohol probably increased after the law change.
