this article, Jensen et al. challenge the conventional
demographic interpretation that reductions in fertility
in industrialized nations can be explained completely by changes
in the choices that women are making about whether and when to bear
et al. explore instead the possibility that part of the
decline in fertility rates in the industrialized world that has
occurred over the past several decades is a result of long-term
trends in male reproductive health, particularly sperm density and
quality. They propose the possibility that fewer babies
are born in part because couples are having greater difficulty conceiving.
et al. do not argue that reductions in male reproductive
health are the sole contributor to the demographic trends,
nor do they conclude with certainty that reductions in
sperm quantity and quality are involved, but they make a plausible
case for the possibility that sperm declines are contributing to
changes in industrialized country birth rates.
substantiated by further research, this is a profoundly
important reinterpretation of conventional demographic wisdom. Jensen
et al.'s provocative arguments warrant further investigation,
not just in Denmark but in other industrialized countries that are
experiencing similar declines.
rates have declined dramatically in industrialized countries to
the point now that in many countries the birth rate is below the
level (absent immigration) required to sustain population size,
generally accepted to be 2.1 children per couple. In Italy and Spain
it has fallen as low as 1.2 children per couple. These declines
below replacement are causing significant changes in social conditions
because they lead to a change in the age structure of the population.
Ageing populations have fewer entry level workers and more senior
citizens, putting severe strains on socio-economic systems.
conventional interpretation of the drop in fertility in industrialized
countries is that women are making different choices about child-bearing,
wanting fewer children and waiting until later in life to have them.
This is seen as a response to several factors, mostly economic:
increases in child survivorship, thought to encourage fewer children
because the likelihood that any one will reach adulthood has increased.
economic utility of children in urban vs. rural settings. As the
world urbanizes, the economic value of children as family farm
out-of-home economic opportunities for women, made possible by
changing societal values and made conspicuous by modern electronic
communication; out-of-home employment competes directly with child-rearing
and increases the value of time that would otherwise go to child-rearing.
et al. do not argue that these factors don't contribute.
Rather, they propose that falling male fertility may be contributing
question is whether the decreasing number of births can be
attributed to changing social structures alone. It appears
that we should also consider the possibility that decreased
fecundity (ability to conceive) may contribute to the decreased
essence, while some of the decline in fertility (birth rate) may
be due choices by women, a portion may be due to declines in fecundity
(the ability to conceive). The former is voluntary, the latter involuntary.
conjecture arises from a startling medical discovery, published
in 2000, that a substantial fraction (roughly 30%) of 19-20
yr old men in Denmark have sperm densities so low as to likely impair
fertility. Further, there has been a steady decline in sperm quality
in Denmark underway for several decades. Related to this, as indicated
in the graph below (from Jensen et al. 2002), men born
later in the 20th century have lower sperm counts than those born
from Jensen et al.
explore their hypothesis further, in this paper Jensen et al.
examine fertility rates in Danish teenage women, arguing that this
age group of women are unlikely to be experiencing the same economic
forces that might affect older women because most pregnancies in
the younger age group are not sought or planned but instead are
actively avoided. Teenage pregnancies in Denmark for the most part
are unplanned and the result of inadequate contraception. Independent
studies of Danish sexual practices cited by Jensen et al.
had already indicated that for at least the past decade, teenage
sexual behavior in Denmark has been relatively unchanged: no overall
pronounced changes in sexual activity nor use of contraceptives.
pregnancy rates in Denmark have nonetheless been in slow but steady
decline for at least a decade, consistent with what would be expected
on the basis of concurrent changes in sperm quality and the large
fraction of young Danish men whose sperm counts are low enough to
from Jensen et al.
et al. also summarize data on desired vs. actual family
sizes in Denmark, and on the increasing frequency of medical interventions
to achieve pregnancy:
data on desired vs. actual family sizes came from a study
in which young women were asked about their desired family size
and that size was then compared, later in their life, to the actual
family size they achieved. Achieved family sizes were lower than
Jensen et al. calculate that up to 5% of all births in
Denmark now involve some form of medical treatment to assist fertilization.
They caution that while this may be due in part to reductions
in male reproductive ability, it may also be due to increasing
availability and increasing success of these interventions. These
causes are not mutually exclusive.
summary, Jensen et al. combine information from several
different sources to challenge the conventional demographic interpretation
of why fertility rates are falling in industrialized countries.
Instead of the decline being a result solely of voluntary choices
by women about how many children they should have, involuntary factors
may also be involved, specifically the increasing percentage of
men whose sperm density is sufficiently low to impair fertility.
They encourage others to examine this possibility:
to the concern caused by the low sperm counts among Danish
men, the Danish Ministries of Health and Environment have
launched a surveillance programme which includes an annual
examination of the semen quality in 600 young Danes from the
general population. We propose that researchers in other countries
with low and falling fertility rates among young women should
consider the possibility that semen quality of their younger
male cohorts may also have deteriorated."