One of the hardest tasks for a marketer is to have the subject line of your email heard (i.e., read) over all the noise of an inbox, which is why email campaign managers turn to the always-effective coupon.

Killer Visual Strategies (formerly Killer Infographics) found that in 2013 there was a 50% year-over-year increase in campaigns mentioning coupons in emails. Coupons were also the No. 1 mobile email campaign trend.

Moreover, strong evidence shows that coupons are much more effective than promotional mailings: Coupons have a 34% higher unique click rate and a 48% higher revenue per email rate.

Offering coupons is all about giving customers what they want—deals. Nearly three-fourths of brand loyalists and over 50% of the general population seek out special offers.

When a consumer finds that perfect deal, oxytocin and happiness levels increase in his or her brain. Good email marketing is all about finding a way to flip that happy switch in consumers' minds.

Check out the infographic to find out more about how using coupons in email campaigns will increase the ROI of the campaign: