Jensen, TK, Carlsen, E, Jørgensen, N, Berthelsen, JG, Keiding, N, Christensen, K, Petersen, JH, Knudsen, LB and Skakkebæk, NE. 2002. Poor semen quality may contribute to recent decline in fertility rates Human Reproduction 17(6): 1437-1440.

In 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 children.

Jensen 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.

Jensen 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.

If 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.

Birth 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.

The 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:

• Long-term increases in child survivorship, thought to encourage fewer children because the likelihood that any one will reach adulthood has increased.
• Decreased economic utility of children in urban vs. rural settings. As the world urbanizes, the economic value of children as family farm labor decreases.
• Increased 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.

Jensen et al. do not argue that these factors don't contribute. Rather, they propose that falling male fertility may be contributing also.

In 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.

Their 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 earlier.

adapted from Jensen et al.

To 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.

Teenage 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 impair fertility.

adapted from Jensen et al.

Jensen et al. also summarize data on desired vs. actual family sizes in Denmark, and on the increasing frequency of medical interventions to achieve pregnancy:

• The 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 desired.
• 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.

In 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: