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[For
an overview of where the sperm debate stands, click here.]
Swan,
SH, EP Elkin and L Fenster. 2000. The Question of Declining Sperm
Density Revisited: An Analysis of 101 Studies Published 1934-1996.
Environmental
Health Perspectives 108:961-966.
In
1992, Carlsen
et al. reported a large, global decline in sperm count
for the period 1938-1990 based on 61 reports of sperm count from
different countries around the world during that period. In 1997,
Swan et al. reanalyzed
the same data set using multivariate statistics. This reanalysis
confirmed Carlsen et al., subject to constraints
inherent in retrospective longitudinal studies of sperm count.
This
new study increases the sample size by 2/3rds compared to the original
paper, first by adding papers published subsequent to 1990, then
through addition of a series of papers revealed by additional reviews
of existing literature. A few of the studies used in Carlsen et
al. were deleted from the analysis for reasons detailed in the
current paper.
Swan
et al. reason in this paper that if the finding by Carlsen et al.
or by their earlier paper were a result of biases in the original
sample, then adding expanding the data set by 2/3rds with new sample
points should eliminate or at least reduce the statistical strength
of the conclusion. In fact, the original finding is upheld, with
a decine in sperm remaining highly significant statistically in
the expanded data set. They conclude that the original result was
not produced by a biased sample, nor dependent upon any one or a
small number of data points included in the analysis.
What
do they conclude?
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The
current analysis suggests that the previously reported trends
have continued, at least until 1996. We have also shown that
the studies initially used by Carlsen et al. (1) did not represent
a biased selection of the English language literature. Nevertheless,
it is likely that neither this publication nor further statistical
analyses of historical data will resolve the continuing debate
over declining sperm counts. Critics will continue to challenge
the reliability of historical data, and most will agree that
residual confounding, which may be appreciable, cannot be completely
eliminated. |
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They
go on to observe:
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The
entire issue of declining sperm count has gained in importance
because of the recognition of several other trends that reflect
a decline in male reproductive health. Testicular cancer incidence
has increased significantly for at least the past 20 years in
most of the Caucasian populations that have been studied. Trends
in rates of cryptorchidism are consistent with those for testicular
cancer, for which cryptorchidism is a significant risk factor.
These increases in rates of testicular cancer and male genital
tract abnormalities, like decreasing sperm density, have primarily
been seen in Western countries. Several authors have suggested
that these trends, together with decreases in semen quality,
may reflect a more generalized increase in testicular dysfunction.
Although few of these trend studies have examined possible causes,
common environmental exposures are plausible. If environmental
factors have produced some, or all, of the temporal changes
in sperm density, the regional differences that have been reported
in semen quality, even within countries, may also reflect variation
in these environmental factors. |
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