Using threats to get children to follow instructions or be obedient is nothing less than assuming them to be robots. Threats in my experience are counterproductive, yet they continue to be a tool being deployed by parents wanting their children to finish a meal, behave in public or to complete their studies or homework quickly.
Some of the threats I have come across are, study or else a ghost will come, finish your food quickly or else a strange uncle will take you away, behave yourself or the police will come and catch you — possibly some of the most common ones doing the rounds.
If one were to step back and think, these threats are doing nothing but scarring the child for life. They are likely to develop fears and may also develop possible psychiatric issues, which may play out at a latent level or be witnessed through mannerism and behaviours. The kids are too young to rationalize the threat and will only react in fear.
A more practical approach, which requires patience and tact, could be to break the situation into questions and then answering the same to sensitize the child. This would be more impactful in making the child realize that what they are doing is wrong and hopefully set a precedence. For example if we were facing a situation wherein the child is not eating his or her food, some possible questions around how does the body get energy, what happens when the body is energy deprived, etc., would certainly help in the child understanding the importance of correct and timely nourishment.
So as parents, next time we decide to deploy threats to get what we want the child or children to do, remember, we are possibly creating a problem. Pause, and put yourself in the shoes of the child and think how you would react if someone with power were to pull the same threat on you.