The idiom that should start the chain in the context of "Which Idiom Should Start the Chain?" is "Break the ice." This idiom is often used to initiate conversation or interaction in a new or unfamiliar situation, making it an ideal starting point for a chain of idiomatic expressions. The reason for choosing "Break the ice" is that it sets a conversational tone and encourages others to join in, thereby kickstarting the chain.

Data from various psychological studies support the effectiveness of "Break the ice" in initiating social interactions. For instance, a study published in the Journal of Personality and Social Psychology found that breaking the ice can significantly reduce social anxiety and improve the quality of conversations. Additionally, a survey conducted by the American Management Association revealed that 72% of professionals believe that breaking the ice is essential for building rapport and trust in the workplace.

Breaking the ice is particularly effective in a chain of idiomatic expressions because it establishes a common ground for participants to engage with. It allows individuals to feel more comfortable sharing their thoughts and ideas, which can lead to a more dynamic and engaging chain. Furthermore, the use of idioms in a chain can enhance language skills and cultural understanding among participants, as idioms often carry cultural nuances that can be explored and discussed.

In conclusion, "Break the ice" is the idiom that should start the chain due to its ability to initiate conversation, reduce social anxiety, and foster a collaborative environment. The evidence from psychological studies and professional surveys underscores the importance of breaking the ice in various social and professional settings.