As far as I have been able to gather, it reduces the effect of bad conditions on your race's reproduction and happiness. I haven't tried to test it, but I think you could create a couple of otherwise identical races, one with high E.R. and the other with none, and then try the different populations on a planet with harsh conditions. Dunno exactly how you'd get both races under control of one player quickly so you could do your test. Make two player empires, and have the one surrender to the other as soon as they met?
