By the end of the 1970s and the early part of the 1980s, the federal government once again became concerned about the minimum legal drinking age within each state. The government knew that an amendment to the US Constitution would likely not work because of the past (e.g. 18th Amendment failed). So, the federal government came up with an even cleverer way to raise the national minimum drinking age through the use of an "unfunded federal mandate."

President Ronald Reagan signed into effect the National Minimum Drinking Age Act (NMDAA) in 1984. The NMDAA is an example of an "unfunded federal mandate." A "federal mandate" is a federal law that orders the states to do (or not do) something or face the possibility of being penalized in some other way. Federal mandates can also be either be "funded" or "unfunded." A funded federal mandate gives the states money to assist them in carrying out the mandate. An unfunded federal mandate does not give the states any money. In other words, the states have to enforce the unfunded federal mandate without any additional money from the federal government.

Under the NMDAA, the federal government ordered the states to increase their minimum legal drinking age to 21 years old. States were free to say "no," but for any state that did so it would lose approximately 10% of its federal funding for highway public transportation under another act called the Federal Aid Highway Act. Therefore, the federal government mandated the states to pass laws increasing their minimum drinking age to 21 years old or face losing money for their highways. The NMDAA was also "unfunded" because the federal government did not give the states any money to help in enforcing the new minimum drinking age law. A number of states initially said "no" to NMDAA, but by 1988 every state passed a law raising its minimum drinking age to 21 in order to continue receiving all the federal money they were entitled to under the Federal Aid Highway Act.

Unfunded federal mandates have generally been met with much hostility from the states, but they are constitutional in the eyes of the law. This is because the federal government is allowed to "attach strings" to any law/mandate that is passes. In other words, the federal government can say to the states:

So, why did the federal government decide on ‘21’ as the magic number for the minimum drinking age? Next, we’ll find out.