From talking to other parents, it seems like most boys (children?) go through an early stage of hitting at around 1. I have a 14-month old (2nd) son who occasionally hits me in the face, and occasionally hits his big brother in the head with objects. His big brother is very good about not hitting back, and playdates have been pretty well monitored, so we're fairly sure no one is modelling this behaviour.

With the older son (who is now 5), we successfully discouraged this at the ~1-year-old stage by sternly saying "no-hitting" and grabbing his hand as soon as he did this. In retrospect, we believe our 1st seems to have been unusually well-behaved at this age (compared to our 2nd and anyone else's child we knew at 1 year old)

Our 2nd is perhaps a more typical boy, and shows much more signs of testosterone/aggressiveness, and challenges us and tests his boundaries much more often than our 1st did. The stern "no-hitting" doesn't seem to work so well, even when we follow up with a brief time-out (in the playpen, which is now almost strictly used for this purpose).

We've heard that all kids go through this phase, and that the more you make of it, the longer it lasts.

Does anyone have good advice on what do here to ensure this doesn't develop into something worse?
Should we just stay on track with the stern "no-hitting"/time-outs?
Or should we ramp up the punishment somehow? Or ignore it in the hopes he'll soon grow out of it?

Any research on this topic is especially appreciated.