This is an excellent example of why inferring causation from an observed correlation requires great care. Note that the study did not randomly assign valedictions to emails and observe the causal impact on response rate. Rather, they observed a correlation between the sign-off chosen and the response rate. "Thanks" garners many responses because people use it in emails that make reasonable requests with a good chance…
>"I am, &c.," I've never heard that before, is it cultural/regional? (Compared to US)
https://archive.org/details/lettersjohnson01hilluoft/page/n2...