This has been explained by sex differences in genetic and biological makeup, with boys being biologically weaker and more susceptible to diseases and premature death.[11][12] Recent studies have found that numerous preconception or prenatal environmental factors affect the probability of a baby being conceived male or female. It has been proposed that these environmental factors also explain sex differences in mortality.[13] In most populations, adult males tend to have higher death rates than adult females of the same age (even after allowing for causes specific to females such as death in childbirth), due to both natural causes such as heart attacks and strokes, which account for by far the majority of deaths, and also violent causes, such as homicide and warfare, resulting in higher life expectancy of females. For example, in the United States, as of 2006, an adult non-elderly male is 3 to 6 times more likely to become a victim of a homicide and 2.5 to 3.5 times more likely to die in an accident than a female of the same age.[14] Consequently, the sex ratio tends to reduce as age increases, and among the elderly there is usually a greater proportion of females.