Found this by a web search:
http://search.washingtonpost.com/wp-srv/WPlate/1997-11/24/075l-112497-idx.html
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Biology: More Evidence of Sperm Count Decline
By Joby Warrick
Monday, November 24, 1997; Page A02
The Washington Post
A new study has produced further evidence of a steep decline in sperm
counts among American and European men.
Three California Health Services Department scientists reviewed data
from 61 studies of sperm density over the past half-century. They adjusted
for differences in data collection and other factors that may have skewed
earlier analyses.
Average sperm counts among healthy American men dropped by 1.5
percent each year between 1938 and 1990 -- a decline more rapid than
previous studies have suggested, they found. The decline among European
men was about twice as fast, or 3.1 percent a year, according to a report in
the November issue of Environmental Health Perspectives.
[...]