30% – Greed / Interest in serving own interests

30% – Ego / Belief in own cleverness when wheeling and dealing

30% – Political agenda / Ideology to be pushed forward at all costs, and at all opportunities

10% – Other. In rare cases, this may even contain altruism. / More often, merely self-delusion that it is altruism. Distributed among the other categories.

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As you can see, there is very little chance that a politician will vote on any measure on its own merits. There may be those who believe otherwise. One could do a similar breakdown on them.