In cooperation with researchers from Cornell University, Facebook randomly selected almost 700,000 users for an experiment in “emotional contagion.” Users’ news feeds were manipulated (without their knowledge) to selectively show postings from their friends that were either more positive or more negative in tone, and the emotional tone of their own subsequent postings was measured. The researchers found evidence that people who read emotionally negative postings were more likely to post messages with a negative tone, whereas those who read positive messages were more likely to post messages with a positive tone. The research was widely criticized for being unethical. Explain why.