The evolution of senescence is often explained by arguing that, in nature, few individuals survive to be old and hence it is evolutionarily unimportant what happens to organisms when they are old. A corollary to this idea is that extrinsically imposed mortality, because it reduces the chance of surviving to be old, favors the evolution of senescence. We show that these ideas, although widespread, are incorrect. Selection leading to senescence does not depend directly on survival to old age, but on the shape of the stable age distribution, and we discuss the implications of this important distinction. We show that the selection gradient on mortality declines with age even in the hypothetical case of zero mortality, when survivorship does not decline. Changing the survivorship function by imposing age independent mortality has no affect on the selection gradients. A similar result exists for optimization models: age independent mortality does not change the optimal result. We propose an alternative, brief explanation for the decline of selection gradients, and hence the evolution of senescence.

BackgroundReducing lifespan inequality is increasingly recognized as a health policy objective. Whereas lifespan inequality declined with rising longevity in most developed countries, Danish life expectancy stagnated between 1975 and 1995 for females and progressed slowly for males. It is unknown how Danish lifespan inequality changed, which causes of death drove these developments, and where the opportunities for further improvements lie now.MethodsWe present an analytical strategy based on cause-by-age decompositions to simultaneously analyze changes in Danish life expectancy and lifespan inequality from 1960 to 2014, as well as current Swedish-Danish differences.ResultsStagnation in Danish life expectancy coincided with a shorter period of stagnation in lifespan inequality (1975–1990). The stagnation in life expectancy was mainly driven by increases in cancer and non-infectious respiratory mortality at higher ages (−.63 years) offsetting a reduction in cardiovascular and infant mortality (+ 1.52 years). Lifespan inequality stagnated because most causes of death did not show compression over the time period. Both these observations were consistent with higher smoking-related mortality in Danes born in 1919–1939. After 1995, life expectancy and lifespan equality increased in lockstep, but still lag behind Sweden, mainly due to infant mortality and cancer.ConclusionsSince 1960, Danish improvements in life expectancy and lifespan equality were halted by smoking-related mortality in those born 1919–1939, while also reductions in old-age cardiovascular mortality held back lifespan equality. The comparison with Sweden suggests that Denmark can reduce inequality in lifespans and increase life expectancy through a consistent policy target: reducing cancer and infant mortality.Electronic supplementary materialThe online version of this article (10.1186/s12889-018-5730-0) contains supplementary material, which is available to authorized users.

Background: Diabetes reduces semen quality and increasingly occurs during reproductive years. Diabetes medications, such as metformin, have glucose-independent effects on the male reproductive system. Associations with birth defects in offspring are unknown.Objective: To evaluate whether the risk for birth defects in offspring varies with preconceptional pharmacologic treatment of fathers with diabetes.Design: Nationwide prospective registry-based cohort study.