The "Thumbs Up" gesture is an example of what is known in body language vernacular as an "Emblem". An emblem is a nonverbal display, most commonly made with the fingers, hands and /or arms (but they can be displayed with the parts of the face, shoulders, even legs/feet, etc.) which have very specific meanings within a city, region, country or even an industry/profession. Care must be taken not to misinterpret an emblem - for they can indicate completely thoughts & emotions in different cultures, etc. Some emblems have meanings which are universal or nearly so.
The "Thumbs Up" emblem - indicates "optimism", "good luck", "good performance" or "okay" in North America - while in parts of Europe it can simply mean "one". When given with an upward thrust it means "up yours!" in Australia and when given with a forward thrust it has a similar meaning in Greece. In Japan it can mean "five" or "Man" (Pease).
The vast majority of the time, the thumbs up is a very conscious display - particularly when shown in a typical position for that region/country and emotion. However if we look at the face we often may see conflicting nonverbal signals. In the image above, in addition to the obvious thumb display (consciously), Mitt Romney is also showing a classic facial nonverbal of disgust (unconsciously) after his third Presidential debate with Barack Obama. When two or more body language signals are seen simultaneously and there is conflict in their meanings - the one(s) which arise out of the subconscious are indicative of the true emotional tone.
See also:
Nonverbal Communication Analysis # 2106: Rahm Emanuel, Chicago Teachers' Union Strike and a Body Language with Multiple Meanings
Nonverbal Communication Analysis # 343: Nicolas Sarkozy's Clandestine Thumbs Up - an Emblematic Slip
See also:
Nonverbal Communication Analysis # 2106: Rahm Emanuel, Chicago Teachers' Union Strike and a Body Language with Multiple Meanings
Nonverbal Communication Analysis # 343: Nicolas Sarkozy's Clandestine Thumbs Up - an Emblematic Slip
Nonverbal Communication Analysis # 2155: Body Language Tells at the Second Presidential Debate - Mitt Romney at Attention & President Obama's Disapproval-Subtle Contempt
Nonverbal Communication Analysis # 1652: Crude Oil, Speculators and Counting with Thumb First
Nonverbal Communication Analysis # 2161: Body Language of the Third Presidential Debate - Barack Obama and Mitt Romney - What Did They Do With Their Hands When They Weren't Talking - and What Does This Tell Us?
Nonverbal Communication Analysis # 2152: Second Presidential Debate - Barack Obama's Duping Delight - Benghazi, Libya Attack was an "Act of Terror" - Alpha, Beta, Confidence & Acquiescence
___________________________________________________________________________________