Mother Nature has always ensured that male births outnumber female ones, but the gap has been gradually narrowing over the past three decades in the U.S. and Japan, according to a new study.

Researchers suspect the decline in male births can be explained, at least in part, by paternal exposure to environmental toxins, such as certain pesticides, heavy metals, solvents or dioxins -- chemical byproducts produced during incineration or the manufacture of other chemicals.

Traditionally, it's been expected that for every 100 girls born, there will be about 105 boys. But since 1970, the U.S. and Japan have experienced a downward shift in this male-to-female birth ratio, researchers report in the online edition of the journal Environmental Health Perspectives.

In the U.S., the proportion of boys dropped from 105.5 per 100 girls in 1970 to 104.6 in 2001; in Japan, the male-to-female ratio dropped from 106.3 boys for every 100 girls to just fewer than 105 per 100.

The changes may seem small, but the study authors suspect they are one manifestation of the effects of environmental pollutants on the male reproductive system.

The decline in male births has occurred "at the same time that we've been seeing other signs that male reproductive health is in danger," said lead study author Dr. Devra Lee Davis, a professor of epidemiology at the University of Pittsburgh Graduate School of Public Health.

These other signs, she said, include lower testosterone levels and sperm counts, as well as increases in testicular cancer, a disease that most often affects young men.

Environmental toxins may be a common denominator here, according to Davis and her colleagues. Such exposures may specifically lower rates of male, rather than female, births for a few reasons. They may, for example, affect the viability of sperm that bear the Y chromosome, which determines male sex -- or the viability of male fetuses.

Davis's team found that while fetal deaths have declined overall in recent decades, the proportion of male deaths is growing. In Japan, in particular, male fetuses accounted for about two thirds of all fetal deaths in the 1990s.

Over the years, there have been a number of reports showing that heavy exposure to certain pollutants may affect a man's likelihood of fathering a son.

从人类的自然发展史来看，男孩的出生数量总是多于女孩，但据一项最新调查显示，美国和日本的男女出生数量差距在过去三十年中逐渐缩小。

研究人员怀疑，男孩出生数量的下降一部分是由于父亲与杀虫剂、重金属、溶剂或二恶英等环境中的有毒物质的接触造成的，这些有毒物质是在燃烧或制造其它化学物质过程中所产生的化学副产品。

一般来说,女孩和男孩的出生比约为100比105。但据研究人员在环境健康观察期刊在线版上发表的报告显示，从1970年开始，美国和日本的男女出生比率开始下降。

美国的男女出生比率从1970年的105.5:100下降到了2001年的104.6:100；而日本的这一比率则从1970年的106.3:100跌至不足105:100。

可能这些变化看起来微不足道，但研究人员提出，这是环境污染对男性生殖系统所造成的危害的一种表现。

匹斯堡大学公共卫生研究生院的流行病学教授、研究报告的主要撰写者德芙拉·李·戴维斯博士说，除男孩出生数量下降外，“我们还发现了男性生殖健康受到危害的其它信号。”

据德芙拉博士介绍，这些信号包括睾丸激素水平和精子数量的下降，以及年轻男性易患的睾丸癌发病率的增长。

据戴维斯博士和她的同事们介绍，环境中的有毒物质可能是造成男孩出生数量下降的共同原因。由于一些原因，接触这些物质可能会导致男孩--而不是女孩，出生数量的下降。比如，这些物质会影响含有决定男性性别的Y染色体的精子的生存能力或男性胎儿的发育。

戴维斯领导的研究小组发现，虽然胎儿死亡率在近几十年来总体上有所下降，但男性胎儿的死亡比例却在上升。这种状况在日本尤为突出，上世纪90年代，日本男性胎儿的死亡率占所有胎儿死亡的三分之二。

近些年来，有很多研究报告表明，过多接触某些污染物质会影响男性生儿子的几率。