The democratic peace theory holds that liberal democracies rarely go to war against each other. The first to espouse this idea was the German philosopher Immanuel Kant, who posited that constitutional republics engender peace, because the majority of people will not vote to go to war unless in self-defence. More recently, the democratic peace theory was put forth in 1964 by Dean Babst. Babst carried out the first statistical research into the theory, thus scientifically proving that democracies rarely fight each other. Despite an undeniable statistical correlation between democracy and peace, the democratic peace theory is highly debated amongst political scientists. The definitions of “democracy” and “war” are one contentious issue. Some opponents of the democratic peace theory point to exceptions, such as the Spanish-American War. However, the main criticism of the theory is that it is based on flawed logic – that peace between democracies is not caused by the democratic nature of those states. Furthermore, opponents argue that democracies frequently attack non-democracies, dispelling the notion that democracies are inherently pacifistic. There are several derivatives of the democratic peace theory, including the economic peace theory, which states that increased economic exchange between states helps to avoid conflict.