The participation gap across groups
Voter turnout is not evenly distributed across the electorate. Across most democracies, and consistently in United States data, turnout rises with age, income, and educational attainment. Older voters typically participate at much higher rates than the youngest eligible voters, and college graduates vote at substantially higher rates than those without a high school diploma.
This uneven pattern is the participation gap: the people who actually cast ballots are systematically older, wealthier, and more educated than the eligible population as a whole.
Why turnout varies and why it matters
Several reinforcing factors drive the gap. Older and higher-income citizens tend to have more residential stability, stronger social networks tied to voting, and lower costs of participation such as flexible schedules and easier registration. Habit also compounds: voting in one election strongly predicts voting in the next.
Because elected officials respond to the people who vote, a skewed electorate can pull policy toward the preferences of high-turnout groups. This is why registration drives, automatic registration, and mail or early voting are studied as levers to narrow the gap.