I think their argument is a little more subtle than that -- the issue isn't really that there are no negative results, but rather that the shape of the distribu

I think their argument is a little more subtle than that -- the issue isn't really that there are no negative results, but rather that the shape of the distribution of results is funny. They're saying you'd expect the results to follow a normal distribution centered at some value. Instead, we see a large number of studies clustered just above zero and a right tail, but no left tail. I think they're arguing that this is because the left side of the distribution is unpublished, and without knowing what it looks like, you can't come to a conclusion.