the British but not among the Northern Californians. This suggests that heavy smokers—who were common in Britain during the 1950s but not in Northern California—were selected against. A set of alleles linked to later menarche was more common among women who lived for longer. An allele called ApoE4, linked to Alzheimer's disease, fell in frequency as carriers tended to not live for very long. In fact, these were the only traits that reduced life expectancy Pickrell and Przeworski found, which suggests that other harmful traits probably have already been eradicated. Only among older people are the effects of Alzheimer's disease and smoking visible. Moreover, smoking is a relatively recent trend. It is not entirely clear why such traits bring evolutionary disadvantages, however, since older people have already had children. Scientists proposed that either they also bring about harmful effects in youth or that they reduce an individual's inclusive fitness, or the tendency of organisms that share the same genes to help each other. Thus, mutations that make it difficult for grandparents to help raise their grandchildren are unlikely to propagate throughout the population. Pickrell and Przeworski also investigated 42 traits determined by multiple alleles rather than just one, such as the timing of puberty. They found that later puberty and older age of first birth were correlated with higher life expectancy. Larger sample sizes allow for the study of rarer mutations. Pickrell and Przeworski told The Atlantic that a sample of half a million individuals would enable them to study mutations that occur among only 2% of the population, which would provide finer details of recent human evolution. While studies of short time scales such as these are vulnerable to random statistical fluctuations, they can improve understanding of the factors that affect survival and reproduction among contemporary human populations. Evolutionary geneticist Jaleal Sanjak and his team analyzed genetic and medical information from more than 200,000 women over the age of 45 and 150,000 men over the age of 50—people who have passed their reproductive years—from the UK Biobank and identified 13 traits among women and ten among men that were linked to having children at a younger age, having a higher body-mass index, fewer years of education, and lower levels of fluid intelligence, or the capacity for logical reasoning and problem solving. Sanjak noted, however, that it was not known whether having children actually made women heavier or being heavier