age at first birth is a highly heritable trait. Human evolution continues during the modern era, including among industrialized nations. Things like access to contraception and the freedom from predators do not stop natural selection. Among developed countries, where life expectancy is high and infant mortality rates are low, selective pressures are the strongest on traits that influence the number of children a human has. It is speculated that alleles influencing sexual behavior would be subject to strong selection, though the details of how genes can affect said behavior remain unclear. Historically, as a by-product of the ability to walk upright, humans evolved to have narrower hips and birth canals and to have larger heads. Compared to other close relatives such as chimpanzees, childbirth is a highly challenging and potentially fatal experience for humans. Thus began an evolutionary tug-of-war. For babies, having larger heads proved beneficial as long as their mothers' hips were wide enough. If not, both mother and child typically died. This is an example of balancing selection, or the removal of extreme traits. In this case, heads that were too large or hips that were too small were selected against. This evolutionary tug-of-war attained an equilibrium, making these traits remain more or less constant over time while allowing for genetic variation to flourish, thus paving the way for rapid evolution should selective forces shift their direction. All this changed in the twentieth century as Cesarean sections (or C-sections) became safer and more common in some parts of the world. Larger head sizes continue to be favored while selective pressures against smaller hip sizes have diminished. Projecting forward, this means that human heads would continue to grow while hip sizes would not. As a result of increasing fetopelvic disproportion, C-sections would become more and more common in a positive feedback loop, though not necessarily to the extent that natural childbirth would become obsolete. Paleoanthropologist Briana Pobiner of the Smithsonian Institution noted that cultural factors could play a role in the widely different rates of C-sections across the developed and developing worlds. Daghni Rajasingam of the Royal College of Obstetricians observed that the increasing rates of diabetes and obesity among women of reproductive age also boost the demand for C-sections. Biologist Philipp Mitteroecker from the University of Vienna and his team estimated that about six percent of all births worldwide were obstructed and required medical intervention. In the United