Are career politician members of parliament (MPs) more or less likely to vote against the party line than their peers? Despite growing interest in the behavior of career politicians across parliamentary systems, answers to this question are marked by considerable theoretical and empirical uncertainty. I derive the two most common (but opposing) behavioral predictions before testing them over all legislative votes of two UK House of Commons terms (2005–15) using multilevel modeling of new and disaggregated data on MPs' occupational backgrounds. The finding that career politicians are more likely to rebel challenges conventional wisdom and provides an important empirical foundation for the ongoing debate.

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