I must disagree... I'm pretty sure that in TOS that was Shatner's real hair. My mom worked in the hairdressing business for years and she can spot toupees and the like miles away; her toupee-busting super-powers are even keener when the perpetrator is on TV. And she'll tell you that was his real hair. You can tell because you can also see that it was thinning even then. Although to hedge my bet I *think* they made him wear one near the end of TOS.
The same pattern happened with Sean Connery in the James Bond films.
And going back to that hair = bad ST; you could sometimes tell that Bakula was a predecesor to Kirk... sometimes under some lighting angle you could tell he's thinning.
Me... I cheated by having hair transplants and taking Propecia. You can laugh now. I'm waaaaaaaaaaaaaaaaaaaaaaaay OT.