A number of biologists have noted the fact that when sexually aroused, the lips become flushed with blood and turn red. It is reasonable to assume that the popularity of lipstick may be partially due to this fact, along with the fact that in experiments, men rate the women wearing lipstick as more attractive. In eye tracking experiments, men spend more time looking at women whose lips are redder.

Lipstick is a visual pheromone - a signal of desire and desirability. And it is no secret that men can be very responsive to visual stimuli. Like chemical pheromones, lipstick alone does not guarantee results. But it is part of the dance.

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The size of the pupils can be another indicator of sexual arousal - see the Aphrodisiology page on belladona. Lipstick can also be a signficant source of exposure to endocrine disruptors.