Nonverbal communication is not necessarily a universal language. The thumbs-up gesture is often used in the United States to indicate agreement and affirmation. The same gesture in Italy means the number one, yet in American Sign Language it means the number 10; and thumbs up in many Middle Eastern countries is considered an obscene, insulting gesture.
Thumbs up is a fairly overt gesture. More subtle is a gentle rocking of the head from side to side, which in many cultures means disagreement. However, the side-to-side gesture in India is seen as a sign of agreement. Misinterpreting this nonverbal cue can often confuse westerners visiting India.