Is it unethical to not tell your date that you are not interested in a long term relationship with them until they start developing feelings for you?

This would really depend on the expectations one's conduct gives rise to. These are initially the expectations that it would be reasonable to have in the society and subculture in question. Thus, if a college student from Montana is spending spring break in Florida and there dating someone from Oklahoma, for example, then the reasonable expectation would be that the relationship is a fling that will not lead to a long-term relationship. On the other hand, if two young Amish people from neighboring villages in Pennsylvania are dating each other, then the reasonable expectation would be that they are contemplating a life-long bond. Most cases obviously are somewhere in-between in that it is somewhat unclear what counts as normal in the relevant context.

It is helpful here that, as the dating proceeds, the two persons may learn a lot about each other and, in particular, about each other's actual expectations. These may differ from the reasonable expectations, which are (roughly speaking) based on statistical probabilities. Our college student from Montana may find to her surprise that her date from Oklahoma is looking for a long-term relationship. That Oklohoma student may harbor the false belief that most college students spending spring break in Florida are looking for a lasting relationship or may harbor the false belief that she and the Montana student share the conviction that dating is meaningful only when there is the firm hope for a long-lasting relationship. Once this dawns on the Montana student, she ought to clarify the situation even if her partner's expectation is, under the circumstances, quite unreasonable. Failing to do this, she would be knowingly misleading the student from Oklahoma.

In other words, it is OK not to tell so long as you have good reason to believe that your partner already understands. In case of doubt, do tell or at least try to find out more about your partner's expectations.

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