Voter turnout is the percentage of eligible voters who cast a ballot in an election. (Who is eligible varies by country, and should not be confused with the total adult population. For example, some countries discriminate based on sex, race, and/or religion. Age and citizenship are usually among the criteria.) After increasing for many decades, there has been a trend of decreasing voter turnout in most established democracies since the 1960s. In general, low turnout may be due to disenchantment, indifference, or contentment. Low turnout is often considered to be undesirable, and there is much debate over the factors that affect turnout and how to increase it. In spite of significant study into the issue, scholars are divided on reasons for the decline. Its cause has been attributed to a wide array of economic, demographic, cultural, technological, and institutional factors. There have been many efforts to increase turnout and encourage voting.
Different countries have very different voter turnouts. For example, in the United States 2008 presidential election turnout was 64% and 68% among African Americans (generally credited to Barack Obama's candidacy). In Belgium, which has compulsory voting, and Malta, which does not, participation reaches 95%. These differences are caused by a mix of cultural and institutional factors.
In any large election the chance of any one vote determining the outcome is low. Some studies show that a single vote in a voting scheme such as the Electoral College in the United States has an even lower chance of determining the outcome. Other studies claim that the Electoral College actually increases voting power. Studies using game theory, which takes into account the ability of voters to interact, have also found that the expected turnout for any large election should be zero.