The more aware of pain and suffering you as a concept the worse the pain and suffering is. Bugs donít suffer as much as a dolphin can as an example. If you canít contemplate death or pain beyond instinct then I think its a lot easier to accept death and pain. Cows for example suffer immensely when they know they are going to be slaughtered. But if you separate them, do it behind closed doors and make sure they canít anticipate being slaughtered they are less upset. Death being instant, there is no time for them to suffer.
Iíd argue that some Down Syndrome people suffer less as well, because some are often ignorant to the horrible things around them.
Perhaps IQ is not a factor, but certainly awareness which is tied in someways to IQ.