Gratification delay....an important life skill. The original "Marshmallow Study " by Walter Mischel (1972) supported the notion that kids who were able to delay gratifica at four had better life outcomes (or higher SAT scores, anyways!). For more on that: http://www.slate.com/blogs/xx_factor/2012/10/16/the_marshmallow_study_revisited_kids_will_delay_gratifcation_if_they_trust.html
This revisit offers more information in support of TEACHING the skills that allow kids to say no to themselves. Additionally, kids who believe waiting will ultimately PAY OFF are more likely to wait.
Teach, teach, teach. It works, even if biology has us leaning in one direction. Additionally, consistent parenting with good follow-through will provide reliable environments that allow kids to learn to wait.