In a series of five studies, Oppenheimer systematically examined the complexity of the vocabulary used in various passages (including job applications, academic essays and translations of Descartes). He then asked people to read the samples and rate the intelligence of the person who allegedly wrote them. The simpler language resulted in significantly higher ratings of intelligence, showing that the unnecessary use of complex language sent out a bad impression.
Excerpt from: 59 Seconds: Think a little, change a lot by Richard Wiseman