In some cases, locally, they can. It's a complex matter, subject to a great number of influences. However, in general, experience has shown that factors like war, disease, famine and above all high child mortality/poverty rates cause human populations to breed more prolifically, in order to counter perceived death rates. Across all human society the experience has been that the more prosperous societies become, the lower their birth rates. Change in birth rates is very slow, however, like slowing down the proverbial supertanker.