Une branche importante et r6cente de la th6orie des probabilit6s, bas6e sur les processus ponctuels, les martingales en temps continu et l'int6gration stochastique, s'est rapidement diffus6e en statistiques appliqu6es pour l'analyse des 6v~nements connus par une mSme entit6: individu, famille, entreprise, etc.., en sciences sociales et 6conomiques; machine, ordinateur, etc.., en fiabilit6 industrielle.Les auteurs dressent ici une vue synth6tique et trSs complete des d6veloppements de la th6orie des processus stochastiques au cours des quinze derni~res ann6es, en les appuyant sur de nombreuses applications pratiques analys6es de fa~on approfondie ~ l'aide de ces m&hodes.Le point de d6part de cette th6orie se trouve dans la publication en 1978 des r6sultats de la th~se d'Aalen sur l'utilisation des processus ponctuels dans les modules d'inf6rence non-param6trique. L'int6rSt d'une telle m6thode est de montrer que de nombreux processus observ6s peuvent se d6com-poser en deux termes: le premier est un processus pr6visible, qui correspond en d6mographie ~ des quotients cumul6s, alors que le second est une martingale. Rappelons ici la signification empirique de ce terme: une martingale est en gros un processus dont la moyenne reste nulle tout au long du temps, quelque soit la strat6gie suivie par l'individu. L'existence de th6or~mes limites pour des processus de ce type permet d'int6grer toutes les approches ant6rieures de l'analyse des biographies dans une th6orie unifi6e d'une tr~s forte coh6rence.I1 est int6ressant pour le d6mographe de voir le r61e pr6pond6rant jou6 ici par une quantit6 que l'on n'avait pas coutume de consid6rer dans les manuels classiques: les quotients cumul6s qui permettent de construire en fonction de la dur6e de s6jour des graphiques de Nelson-Aalen. Tout l'int6rSt de l'utilisation de ces graphiques appara~t lorsque l'on travaille sur des risques comp6titifs, qui ne sont g6n6ralement pas ind6pendants. Dans ce cas, alors que les fonctions de s6jour calcul6es pour chaque risque en supposant les sorties pour autres causes comme des sorties d'observation ne sont comparables que sous l'hypothSse que les divers risques sont ind6pendants, les quotients cumul6s peuvent 8tre consid6r6s comme des processus ind6pen-

Objective-To investigate whether semen quality has changed during the past 50 years.Design-Review of publications on semen quality in men without a history of infertility selected by means of Cumulated Index Medicus and Current List (1930-1965) and MEDLINE Silver Platter database (1966( -August 1991.Subjects-14947 men included in a total of 61 papers published between 1938 and 1991.Main outcome measures-Mean sperm density and mean seminal volume.Results-Linear regression of data weighted by number of men in each study showed a significant decrease in mean sperm count from 113 x 106/ml in 1940 to 66x 106/ml in 1990 (p<0-0001) and in seminal volume from 3*40 ml to 2-75 ml (p=0.027), indicating an even more pronounced decrease in sperm production than expressed by the decline in sperm density.Conclusions-There has been a genuine decline in semen quality over the past 50 years. As male fertility is to some extent correlated with sperm count the results may reflect an overall reduction in male fertility. The biological significance ofthese changes is emphasised by a concomitant increase in the incidence of genitourinary abnormalities such as testicular cancer and possibly also cryptorchidism and hypospadias, suggesting a growing impact of factors with serious effects on male gonadal function.

Objective-To investigate whether semen quality has changed during the past 50 years.Design-Review of publications on semen quality in men without a history of infertility selected by means of Cumulated Index Medicus and Current List (1930-1965) and MEDLINE Silver Platter database (1966( -August 1991.Subjects-14947 men included in a total of 61 papers published between 1938 and 1991.Main outcome measures-Mean sperm density and mean seminal volume.Results-Linear regression of data weighted by number of men in each study showed a significant decrease in mean sperm count from 113 x 106/ml in 1940 to 66x 106/ml in 1990 (p<0-0001) and in seminal volume from 3*40 ml to 2-75 ml (p=0.027), indicating an even more pronounced decrease in sperm production than expressed by the decline in sperm density.Conclusions-There has been a genuine decline in semen quality over the past 50 years. As male fertility is to some extent correlated with sperm count the results may reflect an overall reduction in male fertility. The biological significance ofthese changes is emphasised by a concomitant increase in the incidence of genitourinary abnormalities such as testicular cancer and possibly also cryptorchidism and hypospadias, suggesting a growing impact of factors with serious effects on male gonadal function.

Male reproductive health has deteriorated in many countries during the last few decades. In the 1990s, declining semen quality has been reported from Belgium, Denmark, France, and Great Britain. The incidence of testicular cancer has increased during the same time. Incidences of hypospadias and cryptorchidism also appear to be increasing. Similar reproductive problems occur in many wildlife species. There are marked geographic differences in the prevalence of male reproductive disorders. While the reasons for these differences are currently unknown, both clinical and laboratory research suggest that the adverse changes may be inter-related and have a common origin in fetal life or childhood. Exposure of the male fetus to supranormal levels of estrogens, such as diethlylstilbestrol, can result in the above-mentioned reproductive defects. The growing number of reports demonstrating that common environmental contaminants and natural factors possess estrogenic activity presents the working hypothesis that the adverse trends in male reproductive health may be, at least in part, associated with exposure to estrogenic or other hormonally active (e.g., antiandrogenic) environmental chemicals during fetal and childhood development. An extensive research program is needed to understand the extent of the problem, its underlying etiology, and the development of a strategy for prevention and intervention. Environ Health Perspect 104(Suppl 4): 741-803 (1996)