Sperm output of healthy men in Australia: magnitude of bias due to self- selected volunteers

Controversial claims, based on a meta-analysis aggregating 61 heterogeneousobservational studies, have been made that human sperm output has decreasedby 50% over the last six decades and that this trend may be due to globalpollution. If true, such effects should be evident in all areas of theglobe; however, longitudinal studies within single centres in Europe andAmerica have produced conflicting results and there are no reports from thesouthern hemisphere. We therefore reviewed semen analyses obtained from1980-1995 from 689 healthy men volunteering for screening either aspotential sperm donors for a donor insemination programme (n = 509) or toparticipate in five male contraception research studies (studies no. 1-5, n= 180). All were recruited through the Andrology Unit of the Royal PrinceAlfred Hospital, Sydney, by the same doctors using standard methods ofrecruiting, screening and laboratory examination throughout the period1980-1995. Recruitment was by advertising without regard to marital orfertility status except in two contraceptive efficacy studies (no. 1 andno. 3) where participants had to be in a stable relationship requiringcontraception. Analysing the first semen sample individually or whengrouped by year of ejaculation, there was no significant difference insperm concentration over time or between years or according to year ofbirth. During the second half of this period, 180 consecutive volunteerswere recruited by the same doctors and staff for five male contraceptionstudies. The median sperm concentration for studies no. 1 (103 x 10(6) ml)and no. 2 (142 x 10(6) ml) were significantly (P < 0.05) higher than forstudies no. 3-5 (84, 67 and 63 x 10(6) ml, respectively) and for potentialsperm donors (median 69 x 10(6) ml). The inconsistency of these estimatesillustrates the magnitude of bias (up to 100%) in sperm output that mayoccur in recruiting groups of self-referred volunteers within a singlecentre. This highlights the invalidity of extrapolating similar findings onsperm output of self-selected volunteers to the general male community orin using such study groups to characterize sperm output in supposedly'normal' men.