This inquiry begins with the puzzle of sibling relations. Why are individuals from the same family little more similar in personality than people from different families? Why doesn’t a shared family environment lead to similar values and beliefs? Sulloway suggests a fresh way of understanding how family affects individual development. Among siblings, the most important factor for systematically understanding the sources of individual differences is birth-order. This work shows how birth-order is so fundamental to the family experience that its effects transcend gender, social class, nationality and time. Using historical examples from scientific revolutions, the French Revolution and the Protestant Reformation, this work shows that Marx was mistaken when he located the engine of historical change between families. The engine of history lies in individual differences arising within the family.