Like modern humans, Neanderthals probably descended from a very small population with an effective population—the number of individuals who can bear children—of 3,000 to 12,000 approximately. However, Neanderthals maintained this very low population, proliferating weakly harmful genes due to the reduced effectivity of natural selection. Various studies, using mtDNA analysis, yield varying effective populations, such as about 1,000 to 5,000; 5,000 to 9,000 remaining constant; or 3,000 to 25,000 steadily increasing until 52 kya before declining until extinction. However, all agree on low population, which may have been up to 10 times smaller than contemporary human populations in Western Europe possibly because Neanderthals had much lower fertility rates. Estimates giving a total population in the higher tens of thousands are contested. A consistently low population may be explained in the context of the "Boserupian Trap": a population's carrying capacity is limited by the amount of food it can obtain, which in turn is limited by its technology. Innovation increases with population, but if the population is too low, innovation will not occur very rapidly and the population will remain low. This is consistent with the apparent 150 ka stagnation in Neanderthal lithic technology. In a sample of 206 Neanderthals, based on the abundance of young and mature adults in comparison to other age demographics, about 80% of them above the age of 20 died before reaching 40. This high mortality rate was probably due to their high-stress environment. However, it has also been estimated that the age pyramids for Neanderthals and contemporary modern humans were the same. Infant mortality was estimated to have been very high for Neanderthals, about 43% in northern Eurasia. Neanderthals had more robust and stockier builds than modern humans, wider and barrel-shaped rib cages; wider pelvises; and proportionally shorter forearms and forelegs. Based on 45 Neanderthal long bones from 14 men and 7 women, the average height was 164 to 168 cm (5 ft 5 in) for males and 152 to 156 cm (5 ft) for females. For comparison, the average height of 16 Upper Palaeolithic and Mesolithic modern humans was for males and for females. The fossil record shows adult Neanderthals varied from about in height, though it is possible that some grew much taller. For Neanderthal weight, samples of 26 specimens found an average of for males and for females. Using , the body mass index for Neanderthal males was calculated to be 26.9–28.2, which in modern humans correlates to being overweight.