Engel, Christoph Max Planck Institute for Research on Collective Goods, Max Planck Society;
http://www.sciencedirect.com/science/article/pii/S0165176513005594 (Any fulltext)
Engel, C., & Rand, D. (2014). What Does "Clean" Really Mean? The Implicit Framing of Decontextualized Experiments. Economics Letters, 122, 386-389.