One hypothesis which offers an explanation for this is the accumulation of deleterious mutations over generations, caused by a less harsh environment (e.g. medi

One hypothesis which offers an explanation for this is the accumulation of deleterious mutations over generations, caused by a less harsh environment (e.g. medical care, technology) and advanced parental age, then accelerated by cultural changes [1]. [1] https://link.springer.com/article/10.1007/s40806-017-0084-x https://link.springer.com/article/10.1007/s40806-017-0084-x