Table 1.

Email subject lines tested over the course of the recruitment period.

Round	Type	Content
1	Control	“Please help us understand heart health”
	Social proof	“Join thousands of other Aetna members in a heart health study”
	Exclusivity	“Join a select group of Aetna members in a heart health study”
	Personalized	“(Member name), you're invited to join a heart health study”
2	Control	“(Member name), you're invited to join a heart health study”
	Altruistic	“(Member name), you can help make a difference in medical care”
	New technology	“(Member name), we'd like you to test a new wearable device”
	Personal health information	“(Member name), find out if you have atrial fibrillation, an unusual heart rhythm”