Lam notes decline in seasonal variation in birth rates
"Fact or Fiction?: 'Spring Fever' Is a Real Phenomenon" - Scientific American. 3/22/2007.
"In the late 16th century birth rates typically spiked to 20 percent above the average in March—meaning the babies were conceived in June—but over the past 400 years rates have flattened to about 10 percent above the average, according to research done by David Lam at the University of Michigan's Population Studies Center in Ann Arbor."