Democratic Peace
All democratic peace theories seek to explain the disputed empirical fact that two constitutional democracies have never gone to war with each other in recent history (1816 onwards). As such, they rest on a similar hypothesis: that relations between pairings of democratic states are inherently more peaceful than relations between other regime-type pairings (i.e. democratic versus non-democratic or non-democratic versus non-democratic). To prove the reality of the democratic peace, theorists such as Michael Doyle have sought to show a causal relationship between the independent variable - 'democratic political structures at the unit level' - and the dependant variable - 'the asserted absence of war between democratic states'. Critics, such as Ido Oren, dispute the claims of democratic peace theorists by insisting that there is a liberal bias in the interpretation of 'democracy' which weakens the evidence.