>Granted there are now lots of other things to kill the child, malaria etc, but I guarantee every single person on HN will rather take the chance of malaria than the certainty of not being born in the first place.

Actually no. More people isn't automatically good. Taken to it's extreme this would mean just making as many people as is physically possible with the lowest standard of living possible. There has to be some trade off between number of people vs happiness per person.

Then there is the many worlds thing argument http://lesswrong.com/lw/ws/for_the_people_who_are_still_aliv...