We once did a totally unscientific survey of early head control here on the forum, and anecdotally, it seemed to be a good predictor of HG+ness. Many of us thought it was weird that our kids had early head control, but we'd never put that together with the GTness until so many other people here had seen it in their GT kids, too.
I'd love to see a study on it. I suspect there's some correlation.
The biggest moment for me, though, the moment when I couldn't ignore it with DS8, was when he was about 8 months or so--crawling and cruising, but not yet walking, and not really talking yet. He banged his Hot Wheels car on the wall and I said no. He stopped immediately, then he systematically tested the rule so that he understood the limits for behavior with Hot Wheels cars. He experimented with every permutation of using a car on the wall and the floor--banging and rolling, right hand and left hand--and each time he stopped the second I told him "no," a behavior was off-limits. He was entirely scientific about it. I felt like I was dealing with a much older child.
Once he understood the rule, he followed it from that day on. I never had to explain it again and he never broke it.
I remember having a strong, almost eerie sense that most toddlers probably weren't like that...
With DS5, it was a more recent realization. Just after Christmas 2008, he started doing addition in his head. It was all very sudden, and he moved from addition to addition of large numbers (10s of thousands) with carrying, to subtraction with renaming, and now he's multiplying 2-digit numbers in his his head. It's a big change--I wasn't even thinking he was GT before Christmas!
So DS8 started quick out of the gate and moved steadily all along. DS5 is my leaps-and-bounds kid.
