Much of what we want to know about others is not directly perceivable - are you a nice person? did you really like the cake I baked? would you be a good employee, spouse, president? We rely instead on signals, which are perceivable features or actions that indicate the presence of those hidden qualities. Yet not all signals are reliable. It is beneficial for the con-man to seem nice, for the guest to seem to like the burnt cake, for the unsuitable suitor to seem as attractive as possible. While these deceptions benefit the deceiver, they may be quite costly for the recipient. What keeps signals honest - and why are some signals more reliable than others?