In what sense are pleasant and unpleasant moods "bipolar"? One must differentiate three types of affective bipolarity: static bipolarity (the zero-order correlation between measures of pleasant and unpleasant affect, net of distortions due to measurement error, tends to be strongly negative), dynamic bipolarity (pleasant and unpleasant feelings generally change in opposite directions and to approximately the same extent), and causative bipolarity (the influence of pleasant and unpleasant affect on other variables is approximately equal and opposite). It is argued that static bipolarity is often attenuated by measurement error, dynamic bipolarity can be masked by asymmetrical scaling artifacts, and causative bipolarity is often obscured by both. The experience and influence of pleasant and unpleasant affect may occur along bipolar lines even if the sources of these feelings are understood as physiologically separable systems with distinct neurological loci.