A new study from MasterCard confirms a trend of slowing luxury spending - seen in earnings reports from jewelry sellers Tiffany (TIF -4.4%) and Signet Jewelers (SIG -4.5%) - with sales only increasing 1.8% in April after gaining 6.7% in Q1. Analysts say the supposedly non-cyclical wealth effect isn't holding up against economic headwinds. On watch: COH, KORS, EL, OTCPK:HESAF.