Humans are un/predictable but gain their interest, attract their volatile attention with your unique value proposition. As rule of thumb you might consider the

Humans are un/predictable but gain their interest, attract their volatile attention with your unique value proposition. As rule of thumb you might consider the following: If (Pleasure - Pain > O) then { Purchase == Yes } Interesting read: Stephen Wendel, Designing for Behavior Change - Applying Psychology and Behavioral Economics