Researchers have found overwhelming evidence for genetic effects on behaviors such as sharing
"Yet instances of selfish behavior also abound in society. One recent study used a version of the classic Prisoner's Dilemma, which can test people's willingness to set aside selfish interests to reach a greater good. After modeling different strategies and outcomes, the researchers found that being selfish was more advantageous than cooperating. The benefit may be short-lived however. Another study showed that players who cooperated did better in the long run."
"By comparing identical twins, who share nearly 100 percent of their genes, and fraternal twins, who share about half, researchers have found overwhelming evidence for genetic on behaviors such as sharing and empathy."
"Other work highlights specific genes as key players. My colleagues and I recently identified a gene linked to altruistic behavior and found that a particular variant of it was associated with more selfish behavior in preschoolers."
Ariel Knafo, "Are People Inclined to Act Cooperatively or Selfishly? Is Such Behavior Genetic?" Scientific American Mind, Vol. 25, no. 5 (October 2014): 78, http://www.nature.com/scientificamericanmind/journal/v25/n5/full/scienti....