What factors determine whether people turn out to vote in US elections? Would the use of direct democracy increase turnout or decrease turnout? Why? Should states continue to allow ballot initiatives and other forms of direct democracy? Why or why not?

There are several reasons why people do not vote. People who possess higher levels of education, are in the upper echelon of the socio economic status, and have white collar jobs tend to be more reliable voters. The afore-mentioned characteristics are just a few components of socioeconomic status. Typically those in the higher levels of socioeconomic status are more participatory in voting activities.
Furthermore, social scientists have seen that education level is the strongest predictor of whether or not people vote. The higher the education level, the more people feel engaged in their communities, and in turn, the more they feel that they can affect change by voting.
Obstacles contribute to a decrease in voter participation. For example, Ginsberg, Lowi, and Weir (2011) state, "prior to the 1960's, the widespread use of poll tax and other measures such as the white primary essentially deprived black Americans (and many poor whites) of the right to vote during the first part of the twentieth century." If you could not afford to pay the poll tax then you were essentially shut out of the voting process.
Another aspect of voting that reduces voter turnout has to do with the rules by which people must register to vote. Most states require their residents to register with the state election board before they are able to vote. The criteria that people need to satisfy in order to register to vote reduce registration outcomes especially for those in the lower socioeconomic areas because when registering to vote, the voter must be aware of the registration process and registration deadlines and this is often more difficult for the person wishing to register to vote than the actual act of voting. Furthermore, in states where voters can register on the same day that they vote, or where no voter registration is needed (such as in North Dakota) there is a much higher rate of voter turnout during elections. Over the years, it has become somewhat easier for people to register to vote; however, the removal of further obstacles to vote is necessary in order to ensure that every citizen has easy and open access to their right to vote.
Another obstacle to voter participation in the restriction of the voting rights of people who have committed felony crimes. Most of the states do not allow those who have been convicted of felonies the right to vote. Also, in many of the states, if you are a felon on probation, you still do not have the right to vote. In the 1980's and 1990's the increase in the population of people who have committed felonies meant that there was a decrease in voter turnout. Considering that most convicted felons were African American or Latino, the majority of the population affected by this law was disproportionately black and latino and not white.
Additionally, an obstacle to voting has to do with the fact that most elections are held on Tuesdays. This is a work day for most people and may affect their ability to get to the polls. As a result, many states offer early voting and voting by mail, but according to Ginsburg, Lowi, and Weir (2011) "studies are divided on whether early voting promotes higher turnout."
Finally, direct democracy tends to increase voter turn out because the voter feels that they have a meaningful effect on the election outcome. When voters feel that they can affect a change, they are much more likely to vote. However, because of the need to affect a change, voters who participate in a system that employs direct democracy, are more likely to vote when they are unsatisfied with the current state of events. Voters who participate in a system of direct democracy are least likely to vote when they are happy with the status quo.

In general, voter turnout increases when people are voting in competitive districts or states. When one political party dominates a district or state, people tend not to turn out to vote. In addition, primary elections, off year elections, and local elections tend to have less voter turnout than general elections and elections for state or national office. Voter turnout can also be affected by voting laws and early voting (which has been shown to reduce voter turnout). The demographics of a district also affect voter turnout, as older, wealthier, and whiter districts tend to have higher turnout than less affluent or more diverse or younger districts.
Forms of direct democracy such as initiatives and referendums tend to increase voter turnout because many stakeholders in the election tend to appeal directly to voters and to encourage them to get to the polls. Therefore, these types of direct democracy should be encouraged.