The trader folk wisdom that says “sell in May and go away” proved particularly wise this year. In America, the Dow and S&P 500 indexes recorded their worst May performances since 1940 and 1962, respectively. The global, broad-based MSCI World index shed nearly 10% last month, its worst May performance since the index was created in 1970. A jittery start to June in many major markets suggests that the pain for investors could continue into the summer.