John Matsusaka has written this article for Public Choice. Here is the abstract:

Many political practitioners believe that voters are more likely to approve propositions listed at the top than the bottom of the ballot, potentially distorting democratic decision making, and this belief influences election laws across the United States. Numerous studies have investigated ballot order effects in candidate elections, but there is little evidence for direct democracy elections, and identification of causal effects is challenging. This paper offers two strategies for identifying the effect of ballot order in proposition elections, using data from California during 1958–2014 and Texas during 1986–2015. The evidence suggests that propositions are not advantaged by being listed at the top compared to the bottom of the ballot. Approval rates are lower with more propositions on the ballot.