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 1980s. 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.