presumably Kling would have to argue that the same now-and-then relationship would also apply when exchanging babies born in Stockholm in 1600 and 1740? Remember, there appears to have been at least a 10-fold decline in violence in the intervening time.
I encourage you to read the entire post.
In my original post, what I said was that my most absurd belief is that human nature has changed. I did not say that a change in human nature accounts for all of the cultural improvements that have occurred the past 300 years. My absurd belief is that human nature has changed enough to make a noticeable difference. However, I am not brave enough to suggest that most of the difference in, for example, the propensity toward violence, is due to a change in human nature. Institutions (in Douglass North's broad sense of the term) matter.