Kingdom, one quarter of all births involved the C-section while in the United States, the number was one in three. Mitteroecker and colleagues discovered that the rate of C-sections has gone up 10% to 20% since the mid-twentieth century. They argued that because the availability of safe Cesarean sections significantly reduced maternal and infant mortality rates in the developed world, they have induced an evolutionary change. However, "It's not easy to foresee what this will mean for the future of humans and birth," Mitteroecker told The Independent. This is because the increase in baby sizes is limited by the mother's metabolic capacity and modern medicine, which makes it more likely that neonates who are born prematurely or are underweight to survive. Researchers participating in the Framingham Heart Study, which began in 1948 and was intended to investigate the cause of heart disease among women and their descendants in Framingham, Massachusetts, found evidence for selective pressures against high blood pressure due to the modern Western diet, which contains high amounts of salt, known for raising blood pressure. They also found evidence for selection against hypercholesterolemnia, or high levels of cholesterol in the blood. Evolutionary geneticist Stephen Stearns and his colleagues reported signs that women were gradually becoming shorter and heavier. Stearns argued that human culture and changes humans have made on their natural environments are driving human evolution rather than putting the process to a halt. The data indicates that the women were not eating more; rather, the ones who were heavier tended to have more children. Stearns and his team also discovered that the subjects of the study tended to reach menopause later; they estimated that if the environment remains the same, the average age at menopause will increase by about a year in 200 years, or about ten generations. All these traits have medium to high heritability. Given the starting date of the study, the spread of these adaptations can be observed in just a few generations. By analyzing genomic data of 60,000 individuals of Caucasian descent from Kaiser Permanente in Northern California, and of 150,000 people in the UK Biobank, evolutionary geneticist Joseph Pickrell and evolutionary biologist Molly Przeworski were able to identify signs of biological evolution among living human generations. For the purposes of studying evolution, one lifetime is the shortest possible time scale. An allele associated with difficulty withdrawing from tobacco smoking dropped in frequency among