Linguists have tried to identify potential pronunciation difficulties of nonnative speakers of a language by using contrastive analysis, which was popular in the 1950s
and 1960s. The Contrastive Analysis Hypothesis posits that by contrasting the features of two languages, the difficulties that a language learner might encounter can be anticipated (Crystal, 2003; Fries, 1952). Features of many languages were catalogued by linguists, but it was not possible to systematically predict which areas of English would be difficult for speakers of particular native languages. A less predictive version of the hypothesis was eventually put forth that focused on cross-linguistic influence, which claims that prior language experiences have an impact on the way a language is learned, but these experiences do not consistently have predictive value.