Male hormones drive boyish play
Chemicals in plastics alter the brains of baby boys, making them "more feminine", say US researchers.
Males exposed to high doses in the womb went on to be less likely to play with boys' toys like cars or to join in rough and tumble games, they found.
The University of Rochester team's latest work adds to concerns about the safety of phthalates, found in vinyl flooring and PVC shower curtains.
The findings are reported in the International Journal of Andrology.
Phthalates have the ability to disrupt hormones, and have been banned in toys in the EU for some years.
However, they are still widely used in many different household items, including plastic furniture and packaging.
There are many different types and some mimic the female hormone oestrogen.
This feminising capacity of phthalates makes them true 'gender benders'
Elizabeth Salter-Green, director of CHEM Trust
The same researchers have already shown that this can mean boys are born with genital abnormalities.
Now they say certain phthalates also impact on the developing brain, by knocking out the action of the male hormone testosterone.
Dr Shanna Swan and her team tested urine samples from mothers over midway through pregnancy for traces of phthalates.
The women, who gave birth to 74 boys and 71 girls, were followed up when their children were aged four to seven and asked about the toys the youngsters played with and the games they enjoyed.
They found that two phthalates DEHP and DBP can affect play behaviour.
Boys exposed to high levels of these in the womb were less likely than other boys to play with cars, trains and guns or engage in "rougher" games like playfighting.
There are many different types and the most commonly used are deemed entirely safe by regulators
DEHP - used to make PVC soft and pliable and used in products like flooring
DBP - used as a plasticiser in glues, dyes and textiles
Elizabeth Salter-Green, director of the chemicals campaign group CHEM Trust, said the results were worrying.
"We now know that phthalates, to which we are all constantly exposed, are extremely worrying from a health perspective, leading to disruption of male reproduction health and, it appears, male behaviour too.
"This feminising capacity of phthalates makes them true 'gender benders'."
She acknowledged that the boys who have been studied were still young, but she said reduced masculine play at this age might lead to other feminised developments in later life.
But Tim Edgar, of the European Council for Plasticisers and Intermediates, said: "We need to get some scientific experts to look at this study in more detail before we can make a proper judgement."
He said there were many different phthalates in use and the study concerned two of the less commonly used types that were on the EU candidate list as potentially hazardous and needing authorisation for use.
DBP has been banned from use in cosmetics, such as nail varnish, since 2005 in the EU.
The British Plastics Federation said: "Chemical safety is of paramount importance to the plastics industry which has invested heavily in researching the substances it uses.
"Moreover, the new European Chemical Regulation, REACH, will ensure further rigorous evaluation and testing or chemical substances and their uses."