Why do some people refrain from lying, even when there is no risk of being caught? We design a modified signaling task to study this question, in which subjects choose between being truthful and win small amounts of money or deceitful towards another person and get higher profits. We find that while some people focus on their own personal gain (and avoid lying when potential gains are low), others are more affected by the consequences of their action on their partner (avoid lying when the partner stands to lose large amount of money). We are also examining how activity of different brain regions can explain such individual differences, as well as the general neural mechanism underlying the decision-making process.