The problem is that while unsaturated fats are healthier, they're also tougher to use. They aren't as versatile (as anyone who has attempted to substitute corn oil for butter in a cookie recipe can attest). And saturated fats, apart from being artery cloggers, also tend to go rancid (as anyone who regularly uses lard knows). In the 1950s, researchers realized they could substitute animal fats with vegetable oils by using a hydrogen reaction to turn an unsaturated fat into a partially hydrogenated fat—one with many of the same properties as a saturated fat, but with less, well, saturated fat. Voila, they made margarine.