The Amish are known for their high fertility and large families, but the Amish twinning rate has been of less interest. In an article published in 1970 (over 50 years ago), Cross and McKusick determined the Holmes County, Ohio, Amish twinning rate to be 15.3 twin pairs per 1,000 live births, which ranked among the highest known twinning rates at that time, while the U.S. national twinning rate hovered at around 9 per 1,000 per year. Within a few decades following the Cross and McKusick study, the twinning rates of the American population increased dramatically, and surpassed the Amish twinning rates. This surge in the twinning rate among the general American population was generally accredited to the widespread use of medically assisted reproduction (MAR), which favors multiple births, and the increased birth rates for older women, who have a significantly higher rate of multiple births. Holmes County has eight different Amish sects, ranging from fairly progressive to ultraconservative, and the five largest ones were the subjects of this study. The twinning rates of the different groups vary, parallel with degree of conservatism. The New Order Amish (most progressive) have a twinning rate of 20.1 twins per 1,000 births, whereas the Swartzentruber Amish have a rate of 34.5. Since all the groups are of identical ethnic stock and share virtually identical historical experiences, these twinning variances are difficult to explain, but may be due to diverging dietary practices that paralleled the conservative fragmentation.