The sigmoid is the poserior distribution from Gaussian likelihoods

[Jack Cowan]

Geoff: Apropos Dan Levine's remarks, I introduced the sigmoid in 1965  
at the Wiener Memorial Meeting in Genoa, as a smooth approximation to  
the firing rate vs current curve of a single neuron with a shot noise  
input. (Published in 1968-71).  Later with Hugh Wilson in 1972, 1973  
we redefined the sigmoid as arising in a population of neurons with  
varying thresholds as the integral of a unimodal probability density.

Jack Cowan