Voter turnout is the percentage of eligible citizens who cast ballots in a given election. It can be measured by counting all ballots (including those that are “sign-in” and not actually cast) or by calculating the ratio of voters to the voting age population. (In some cases the latter is a subset of the total citizen population, and in others it excludes noncitizens and ineligible felons).
Political science researchers have long noted that different demographic groups vote at different rates. These differences reflect a combination of economic and social factors. For example, wealthy Americans vote at much higher rates than those with lower incomes. The result of this inequality affects public policy: Politicians are more likely to respond to the desires of their wealthy constituents.
Academic research has also shown that certain demographic groups have a harder time overcoming barriers to participation in electoral politics. These include youth, people of color, and women. In addition, many people vote intermittently; some go from one election to the next without ever casting a ballot.
The dominant theory of why turnout varies year to year focuses on a sort of cost-benefit calculation: Whether it is a financial or personal cost to vote, or a benefit resulting from a win, individual voters weigh these factors in the decision-making process. However, recent scholarship has challenged this theory by showing that voting is often seen as intrinsically rewarding.