Exposure to environmental poisons is suspected of disturbing the secondary sex ratio, i.e. the difference in numbers of girls and boys born as opposed to the ratio of girls and boys conceived. Indeed, several countries, including Canada, Denmark, England and Wales, Germany, The Netherlands, and the USA, have seen the secondary sex ratio shift during the last hundred years or so that the number of girls among live-births has risen significantly. One of the more significant declines is seen in Mexico, although some countries, Ireland, in particular, have seen the reverse, with more boys.
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