made it easier to reproduce. Because taller men and shorter women tended to have more children and because the genes associated with height affect men and women equally, the average height of the population will likely remain the same. Among women who had children later, those with higher levels of education had more children. Evolutionary biologist Hakhamanesh Mostafavi led a 2017 study that analyzed data of 215,000 individuals from just a few generations in the United Kingdom and the United States and found a number of genetic changes that affect longevity. The ApoE allele linked to Alzheimer's disease was rare among women aged 70 and over while the frequency of the CHRNA3 gene associated with smoking addiction among men fell among middle-aged men and up. Because this is not itself evidence of evolution, since natural selection only cares about successful reproduction not longevity, scientists have proposed a number of explanations. Men who live longer tend to have more children. Men and women who survive until old age can help take care of both their children and grandchildren, in benefits their descendants down the generations. This explanation is known as the grandmother hypothesis. It is also possible that Alzheimer's disease and smoking addiction are also harmful earlier in life, but the effects are more subtle and larger sample sizes are required in order to study them. Mostafavi and his team also found that mutations causing health problems such as asthma, having a high body-mass index and high cholesterol levels were more common among those with shorter lifespans while mutations leading to delayed puberty and reproduction were more common among long living individuals. According to geneticist Jonathan Pritchard, while the link between fertility and longevity was identified in previous studies, those did not entirely rule out the effects of educational and financial status—people who rank high in both tend to have children later in life; this seems to suggest the existence of an evolutionary trade-off between longevity and fertility. In South Africa, where large numbers of people are infected with HIV, some have genes that help them combat this virus, making it more likely that they would survive and pass this trait onto their children. If the virus persists, humans living in this part of the world could become resistant to it in as little as hundreds of years. However, because HIV evolves more quickly than humans, it will more likely be