Dilated by maths

The behavioural signal offered by pupil dilation is something I believe has long been under-represented in optical texts. They tend to emphasise the role of the iris as a light limiting stop, and constriction as a way of reducing light input in situations of brightness, of reducing spherical aberration and of increasing depth of focus. As dilation is a sympathetic nervous response, it is not surprising that pupil dilation is indicative of moments of arousal, whether sexual, based on fear (or both, as an ex-grammar school boy like me might suggest!) or as a betrayal of a deliberate aim to deceive. I have further wondered whether the gradual reduction in pupil function over middle to late years has never evolved because the signalling function of the pupil is primarily required during younger years, where sexual attraction is paramount to propogate the next generation. Discuss.