Half Life 2 and Motion Sickness

[Zoo]

For some reason, Half Life 2, specifically makes my friend totally motion sick. Like, throw up after half an hour sick.

And I have absolutely no idea why. A lot of people on the internet have this problem with the Half Life games. They're fine with other games, roller coasters, and lots of other things, but Half Life makes them horribly motion sick. Some people adjust the FOV or FPS, but that doesn't really help. I just find it fascinating. What do the Half Life games have that others don't? I'm really curious. It's a mystery. And I love mysteries.

[Gan]

Here's a list of possible culprits:
Certain frame per second
Camera walking motion
Visual color combination

Motion sickness usually occurs when the equilibrium of your ear fluid doesn't match what your other senses tell you.
Perhaps you are getting too into the game.

[Zoo]

Well, it's my friend. I played straight through each Half Life game by sitting down for several hours. I think it might be the color thing, the speed at which the character moves, and it's possible it could be the FPS. He's running it on an i7+GT 640 2GB, and it's from 2004. So he's probably maxing out his FPS for a 60 Hz monitor. Being that he uses a laptop, there's no way he can try it conveniently on a 75 Hz monitor. Half Life games tend to have dark scenes followed by bright scenes and lots of contrasting color, but that wouldn't explain motion sickness after just a few minutes walking around the same scene. I doubt it's the view bobbing, because he plays Minecraft and Doom, some of the worst culprits of over-zealous view bobbing. I think the one thing that's weird about this is that many, many people have this problem, but only with Half Life games. I wish I had a professional motionsicknessologist to question. XD

[Circuit]

Does your friend have a headcrab attached to his head?  That would explain it.  I recommend a crowbar.