Q:

How is this high, overtone like voice effect done?

Listening to the song That bird has a broken wing by Sun Kil Moon I wondered how they achieved the sound of the voice.
I would describe it as if it had a sort of high, slightly eerie, overtone like layer over the original voice. It sounds also a bit as if there are multiple singers singing the same part.
I found a comparable voice sound in the song Angeles by Elliot Smith and (to a lesser extent) in Carrissa's Weird by Iron and Wine.
Is this some kind of audio effect? Or is there a special way of recording this?

A:

The basic principle is called double tracking.  You can read a good introductory article here: http://www.soundonsound.com/sos/apr09/articles/doubletracking.htm
In the first example there is pitch difference and a strong spatial element being applied.

