This article analyses the predictors of preferential voting in flexible list systems. It does so by focusing on the results of a field experiment carried out in Portugal, an exit poll which took place on election day in 2015. Following the extant literature, and the gaps in the knowledge, we analyse the importance of political sophistication, voting rules and district size for expressing preference votes. Of the four indicators chosen to measure political sophistication, namely education, political interest, gender and age, only political interest is a significant predictor of preferential voting. A ballot which presents no alternative to expressing a preferential vote systematically makes a significant difference, compared to an optional preferential ballot. District size makes a difference only when preferential voting is compulsory, being that the odds of expressing preferences increase to a significant degree in average-sized districts. Finally, political interest tends to lose its significance when preferential voting is compulsory. The article therefore shows that preferential voting does not constitute an obstacle for those with less political sophistication to express a vote when the voting rules make preferential voting compulsory.