Hand Behind Head

He's probably talking about his "Dollie" again.

When nervous, embarrassed, or uncertain—or when lying—a character will sometimes rub the back of his neck with one hand, often with a patently false grin or laugh accompanying the gesture. It's so ubiquitous in anime that it might be easier to list examples where someone doesn't do this.

Related to Face Palm.

Examples of Hand Behind Head include:

Anime and Manga

Tenchi from Tenchi Muyo!
Ranma in Ranma ½.
Negi in Mahou Sensei Negima.
- As well as most of the Baka Rangers, sometimes together.
Most Son males from Dragonball Z.
Occurs occasionally in Fullmetal Alchemist (for example, Maes and Gracia Hughes rub their necks when Ed, Al, and Nina remark upon Gracia's advanced state of pregnancy).
Mai in My-HiME, after Chie asks her about the "crop circle" she created when she arrived at school.
Sasami in the Pretty Sammy series does this fairly frequently when she thinks people have figured out her identity.
Kakashi did this many times in Naruto when trying to brush aside the reasons he was late for practice, or during any other display of Obfuscating Stupidity.
- Naruto does this too, presumably as a homage to Dragon Ball (Naruto is full of them).
- Iruka does it too, especially as a child.
- Naruto's mother, Kushina also does this. Probably where he got it from
In Mahoraba, it appears Shiratori's hand is glued back there.
Both Gourry and Xellos of Slayers do this rather frequently, though with Xellos it's probably for more worrisome reasons.
While Tomo does this more (in the anime) than all the rest of the main and supporting cast combined, the following have also done it: Kagura (the distant second), Chiyo (twice), Yomi, Chihiro, Yukari, and Minamo (once each).
All of the Sailor Moon girls have done this, but it's often done by Mako/Lita when talking to a guy she likes or Usagi/Serena after she's made another silly mistake.
Manabi does this during the reflection meeting in the last episode.
Matsuda of Death Note does this several times, being the bumbling Butt Monkey.
The Nohara males in Crayon Shin-chan.
Inspector Takayama does this early on in the Domu: A Child's Dream: A Child's Dream, embarrassedly asking about supernatural phenomena to a shaman who can tell he's skeptical.
Ichigo from Bleach scratches the nape of his neck whenever he's feeling awkward.

Comic Books

The Phantom Affair, an arc of the X Wing Series comics drawn by Biukovic, Wedge Antilles does this at at least two points.

Fan Works

Between this trope and Motor Mouth, Dan of Thawing Permafrost finds himself doing one or the other on occasion.

Film

James Bond, of all people, does this in GoldenEye, after Valentin Zukovsky calls him a "charming, sophisticated secret agent."

Live-Action TV

Western example: Sheriff Jack Carter, from Eureka.
The Third and Tenth Doctors in Doctor Who.

Video Games

In Phoenix Wright: Ace Attorney, the title character sometimes does this in the court scenes if he makes a mistake. He also does it if he's unsure about something; if, for example, he brings up a piece of evidence without knowing exactly what's going on, he'll do this gesture while explaining his (usually shaky) logic.
- Dick Gumshoe does this pretty often too.
- Phoenix Wright does this as his victory pose in Marvel vs. Capcom 3.
Commander Shepard of Mass Effect has a tendency to do this when modestly acknowledging his/her deeds when admired. Other characters occasionally do it as well.
Spider-Man does this in the Marvel vs. Capcom series and spiritual predecessors.
Ever 17: Kuranari Takeshi's right hand is rarely (if ever) seen. It seems to be permanently fused with the back of his head.
Sora of Kingdom Hearts does this a lot—often around Kairi or Naminé but basically whenever he's thinking about anything. Also, when Goofy scolds him for getting a little too aggressive, his apology involves this gesture.
In The Elder Scrolls III: Morrowind, this was an idle animation that every NPC would eventually play if you waited long enough. (Considering the context, this was suprisingly appropriate.)
Crisis Core: Zack does this frequently enough for it to be an identifying mannerism. Cloud probably picked it up from him. Along with everything else.
In Kid Icarus: Uprising, Pit, Hades, and Viridi are all guilty of this.

Web Comics

Tarvek makes a frequent habit of this in Girl Genius; Gil does this occasionally as well.
Used plenty of times in Suppression mostly by Cerene and Kirk.
Lavender in El Goonish Shive when a tongue-slip reveals how she feels about her boss.

Western Animation

Various characters in Avatar: The Last Airbender.
Disney heroes, most notably Aladdin and Hercules, tend to do this when confronted with attractive and aggressive women. The Beast does it when he's ashamed of himself or frustrated
Kappa Mikey, naturally, in a parody of anime. Guano, Lily and Mitsuki do this on occasion, but the real offender is Gonard!
Jérémie from Code Lyoko pulled this on a couple of occasions.
Hank Hill from King of the Hill does this all the time. It usually happens when Hank has to try to express emotions, which is very difficult for him.
- It got so common that there was eventually an executive order for him not to do it anymore.