Another study showed that exposure to even low levels of chemical pollution while in the womb affects a foetus' development.

In this study involving 207 mother and infant pairs, a correlation was found between the exposure of the foetus to polychlorinated biphenyls (PCBs) and dioxins (PCDFs) and later gender behaviour. Exposure levels were not high, probably background levels in the mothers that they had acquired from their food. Boys exposed to higher levels of PCBs were less likely to engage in masculine patterns of play whilst girls exposed to higher levels of PCBs were more likely to engage in masculine patterns. Both boys and girls exposed to higher levels of dioxins tended to adopt more feminine play patterns.

Exposure during the earliest stages of development (i.e. in the womb) was key. Although the gender behaviours of children exposed to high levels of PCBs or dioxins in breastmilk were affected, these were the same children who had been exposed to higher levels in the womb. This study therefore found no significant link between behavioural change and exposure to PCBs or dioxins through breastfeeding.

An earlier study by another team [1] suggested links between cross gender behaviour in boyhood and a tendency towards homosexuality in later life.

Research on babies born to mothers exposed to PCBs and PCDFs in the Yucheng rice oil contamination incident found reduced spatial orientation abilities in boys, but no effect in girls. (Boys typically perform better in tests of spatial orientation than girls.) The finding was interpreted as demasculinising or feminising effects caused by disturbances in steroid hormones by prenatal exposure to PCBs/PCDFs.

Other studies have found adverse impacts of foetus exposure to PCBs and dioxins on immune system function and neurological development.