Did you know that scientists have been collecting data on sperm
quality for about seven decades? And did you also know that,
according to at least one famous analysis of these data, the
overall quality of sperm has been declining steadily over the
years? During a recent panel discussion, a colleague leaned over
and whispered to me, “What constitutes ‘normal’ today wouldn’t
even have made the cut a few decades ago!” Most of these data
have come from men donating to sperm banks. And most of the
sperm banks are in urban centres in Western countries. This
probably constitutes what we epidemiologists call “selection
bias.” It’s when the way in which the data have been selected
may be affecting the conclusions we attempt to draw from such
data. For example, men who choose to donate are inherently
different from men who don’t. Populations with sperm banks may
be inherently different from populations without them. And
countries with well-organized data collection systems for their
sperm banks are probably different f