"Yes" is similar in meaning to "yeah" and "yea," both casual variants that are probably older than "yes."
Saying 'no' is a refusal skill which is designed to help children avoid participating in high-risk behaviors or other violence, drug, or sex related things. For example the American television advertising campaign Just Say No in the 1980s aimed at spreading awareness about saying "no" to recreational drug use, violence, premarital sex, and any other vices that young people might try.
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