One of the major classes of chemicals that are cited as contributing to male feminisation consists of certain plasticisers, notably phthalates. In 2009, Shanna Swan (professor of obstetrics and gynaecology at the University of Rochester) and her colleagues published a study on the effect of the phthalates DEHP and DBP on expectant mothers and their unborn children. It shows that the boys born from mothers with the highest concentration of these two chemicals were five times less likely to show behaviour typically associated with young males of similar age: 'If the mother's MEHHP concentration of the phthalate metabolites was high, in the upper quartile, the odds that her boy had a score that was less masculine play behavior was five times greater than mothers whose MEHHP was in the lowest quartile." A possible link to sexual orientation is yet to be established.