I'm now fully convinced that the vast majority of toddler boys must walk around with a permanent mild concussion. D spends a large amount of his waking moments running hither and yon as fast as he can, and then either falling and bumping his head or running into an object harder than himself and bumping his head. He's already had his first laceration last month (wandered right off the dock onto a lower level of dock and bit into his lower lip- I had a good 10 minute conversation with myself (and the unfortunate souls in my direct periphery) about whether it was worth it to stitch it up- in the end I opted for letting it heal on its own to avoid the trauma of local anesthesia, and luckily it's healed quite nicely, can't even tell it happened). First laceration at age one; M hasn't had anything deeper than a scrape and she's almost 5! This traumatic event was a couple weeks after he pushed a toy dump-truck and then himself right off the dock into the water. He hardly seemed phased after I jumped in after him a second after hearing the splash. Just coughed a bit and then let me hold him for a few minutes, then off he toddled to play with the dump-truck again. So apparently he has no fear of heights or water. Which will be a good thing later, I suppose, but for now it means I'll be buying him a lighter life-jacket (as the one we have for him is great, but a bit on the heavy side as he can't wear it and get back up on his own if he happens to fall down on his bottom, so I hadn't been putting it on him while he was out of the water and had opted for the constant-surveillance method, but that obviously isn't working so great for me).
So perhaps this permanent-concussion state explains why boys have a tendency to develop more slowly than girls. It's the constant stress to their noggins that they inflict upon themselves.
