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PRD does, in fact, specifically say that fighting defensively is a full round action, BUT, it can also be done as a standard action giving a different TYPE of bonus to AC. But it just looks like the full-round action makes the bonus to AC a Dodge bonus but the standard action gives an untyped bonus to AC, which is probably better, anyway. Very weird discrepancy here, it seems to me... I don't understand this difference at all.
To quote the PRD,
"Fighting Defensively as a Standard Action: You can choose to fight defensively when attacking. If you do so, you take a –4 penalty on all attacks in a round to gain a +2 to AC for the same round.
Fighting Defensively as a Full-Round Action: You can choose to fight defensively when taking a full-attack action. If you do so, you take a –4 penalty on all attacks in a round to gain a +2 dodge bonus to AC for the same round."
Pease go to the PRD and search for "fighting defensively" and explain this difference to me, someone?