This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Every political tradition is partly defined by what it takes from its rivals and what it refuses to take. Social democracy sits in an unusually crowded intersection, close enough to liberalism to be mistaken for it, close enough to communism to have spent a century arguing with it, and close enough to the welfare state as a policy program that the two are often used as if they were synonyms, though they are not quite the same thing.
From liberalism, social democracy borrowed almost the entire political method: free elections, an independent judiciary, civil liberties and the rule of law, none of which nineteenth century socialism could take for granted and none of which social democracy has ever proposed to abolish. What it did not borrow was liberalism's comfort with market outcomes left largely alone. Where classical liberalism treats a competitive market as broadly self-justifying so long as it rests on voluntary exchange, social democracy treats an unregulated market as a source of instability and unearned advantage that democratic government has both the right and the duty to correct, through taxation, regulation and redistribution rather than through liberalism's narrower guarantee of formal equality before the law.
From communism, social democracy inherited a diagnosis without the cure. It agrees, in broad terms, that concentrated private ownership of the means of production gives a capitalist minority disproportionate power over everyone else's working life. It rejects, categorically, communism's proposed remedy: the revolutionary seizure of state power by a vanguard party ruling in the name of a class that never itself gets to vote the party out. Every social democratic party that has held office has done so as one competitor among several, subject to loss at the next election, a constraint communist parties in power have historically refused to accept.
The welfare state is where the confusion is most understandable, because it is genuinely the main instrument through which social democracy has pursued its aims in government. But the welfare state as a policy apparatus, health insurance, pensions, unemployment support, predates social democracy as a named political tradition and has been built by governments across the ideological spectrum, from Bismarck's conservative Germany to Attlee's Labour Britain. Social democracy is the argument for why a democratic society should want a welfare state and how far it should go; the welfare state is the machinery, and a conservative or liberal government can operate the same machinery for very different reasons.