In Berinia, the age at which people could begin to drink alcohol legally used to be 18. In 1990, in an attempt to reduce alcohol consumption and thereby to reduce alcohol-related traffic deaths among Berinians under 21, the legal drinking age was raised to 21. Alcohol-related traffic deaths among people under 21 have decreased significantly since 1990. Nevertheless, surveys show that people in that age-group drink just as much alcohol as they did before 1990.
Which of the following, if true of Berinia, most helps to resolve the apparent discrepancy?