Changing the headline can make a huge difference in your response rate, even if you don't change one word in the rest of the sales letter. (entrepreneur.com)
Judging by the stark differences in responses between this column I wrote in 2011 and this one I wrote last month, I'd guess they're gone within 10 years. (si.com)
Attachment theory has been applied to understand psychological disorders, such as depression, and can explain individual differences in responses to rejection. (journals.plos.org)