> A person with common sense would look at the mass of a baby, and the mass of an adult human.

Weight/mass has very little to do with it. The baby is never 'crushed' it is 'smothered'.

Also when you smother a baby (accidentally or intentional) it squirms. So common sense would indicate the key factor is how heavily you sleep, and how easily could you role onto the baby.

If the babies head is the same level as your head, the chances of rolling on top, would require some pretty intense dreaming.

Knocking a baby off the bed is very unlikely to kill a baby (they are in a relaxed state, and have flexible bone structure/skull).

Research indicates co-sleeping is dangerous when the adult has consumed alcohol or drugs (i.e is sleeping very heavily).

I personally would be a horrible co-sleeper.

I can set 5 alarms and hear none. I only wake up at the end of a sleep cycle, no matter what's happening around me.

Incidentally I just read the child rearing chapter of Jared Diamond's latest book about traditional societies. Turns out they all tend to co-sleep, but usually they sleep on the ground or on hard mattresses. As JD points out, it is easier to notice that you have rolled onto your child with a hard mattress.

This is an excellent point.

I just bought a new mattress and I can say for sure, after having slept on a $100 target futon for the past 2 years, that I'd not know if a baby was underneath me on this one...