In the same way, each individual reasoner is really good at one thing: finding evidence to support the position he or she already holds, usually for intuitive reasons. We should not expect individuals to produce good, open-minded, truth-seeking reasoning, particularly when self-interest or reputational concerns are in play. But if you put individuals together in the right way, such that some individuals can use their reasoning powers to disconfirm the claims of others, and all individuals feel some common bond or shared fate that allows them to interact civilly, you can create a group that ends up producing good reasoning as an emergent property of the social system. This is why it's so important to have intellectual and ideological diversity within any group or institution whose goal is to find truth (such as an intelligence agency or a community of scientists) or to produce good public policy (such as a legislature or advisory board).— Jonathan Haidt, The Righteous Mind: Why Good People are Divided by Politics and Religion
I think that's an excellent case against one-party rule. It also suggests that majority rule, when a single homogenous party is in the majority, is less effective than majority rule with votes requiring cross-party collaboration or at least heterogenous thought within the party.
The focus of the first part of this book is "Intuitions come first, strategic reasoning second," using the metaphor of a rider on an elephant. Haidt offers a lot of evidence that our intuitions and emotions (the elephant) typically make decisions and our consciousness and reason (the rider) is mostly focused on justifying those intuitions. This contrasts, of course, with the Rationalist world view (exemplified by Plato) in which consciousness and reason are in charge and can act independently of intuition. Haidt aligns more closely with Hume and his claim that reason is "ruled by the passions," though Haidt softens the "rules" claim.