解密：美國、日本男孩出生率爲何下降？

解密：美國、日本男孩出生率爲何下降？
Male births declining in the US and Japan

Male births declining in US and Japan

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.

從人類的自然發展史來看，男孩的出生數量總是多於女孩，但據一項最新調查顯示，美國和日本的男女出生數量差距在過去三十年中逐漸縮小。

研究人員懷疑，男孩出生數量的下降一部分是由於父親與殺蟲劑、重金屬、溶劑或二噁英等環境中的有毒物質的接觸造成的，這些有毒物質是在燃燒或製造其它化學物質過程中所產生的化學副產品。

一般來說，女孩和男孩的出生比約爲100比105。但據研究人員在環境健康觀察期刊在線版上發表的報告顯示，從1970年開始，美國和日本的男女出生比率開始下降。

美國的男女出生比率從1970年的105.5:100下降到了2001年的104.6:100；而日本的這一比率則從1970年的106.3:100跌至不足105:100。

可能這些變化看起來微不足道，但研究人員提出，這是環境污染對男性生殖系統所造成的危害的一種表現。

匹斯堡大學公共衛生研究生院的流行病學教授、研究報告的主要撰寫者德芙拉·李·戴維斯博士說，除男孩出生數量下降外，"我們還發現了男性生殖健康受到危害的其它信號。"

據德芙拉博士介紹，這些信號包括睾丸激素水平和精子數量的下降，以及年輕男性易患的睾丸癌發病率的增長。

據戴維斯博士和她的同事們介紹，環境中的有毒物質可能是造成男孩出生數量下降的共同原因。由於一些原因，接觸這些物質可能會導致男孩--而不是女孩，出生數量的下降。比如，這些物質會影響含有決定男性性別的Y染色體的精子的生存能力或男性胎兒的發育。

戴維斯領導的研究小組發現，雖然胎兒死亡率在近幾十年來總體上有所下降，但男性胎兒的死亡比例卻在上升。這種狀況在日本尤爲突出，上世紀90年代，日本男性胎兒的死亡率佔所有胎兒死亡的三分之二。

近些年來，有很多研究報告表明，過多接觸某些污染物質會影響男性生兒子的機率。