This is a question asked by Dr. Paul Zak in an oped piece for TED Talk on cnn.com (here), and supposedly answered. Therefore, its worth the time to address Dr. Zak's argument.
Dr. Zak begins by saying (article):
"The longest debate since humans have been having debates is whether we are good or evil. It underlies the stories of Adam and Eve, Cain and Abel, Jesus and Judas."
First of all, humans are basically evil, despite popular opinion, and I have already covered this issue here, which goes into great detail about the state of humanity post fall. Suffice it to say:
"None is righteous, no, not one; no one understands; no one seeks God. 12 All have turned aside; together they have become worthless; no one does good; not even one." (Romans 3:10-12, ESV)
With the exception of Jesus (see this post for further discussion on the nature of Christ), every human that ever lived falls into the "evil" category.That being said, it is interesting that Dr. Zak uses specifically religious examples at the very outset (trying to make a point?). Next, Dr. Zak asks:
"What is our human nature? Of course, the answer is we can be both good and evil. But what determines which part of our character emerges?"
Human nature was defined above, and as to the answer being both, that's not true. We are evil and when good is done, it is the Imago Dei, the image of God in which we were created (Genesis 1:26), shining through the filth of our wickedness. Dr. Zak here is attempting to address the Theodicy (i.e. the problem of evil), and in one fell swoop define "evil" or "good" in terms of biochemistry. One does not have to be a biochemist to see the difficulty here. For one to define "good" one must have a canon, or measure, to define that which is good. Despite the assertions of existential and relativistic philosophies so prevalent in modernity, "good" or "evil" are not defined by social norms per se. The great philosopher Plato addressed this himself here, and said that to define anything as "good" we must have a perfect "Form" by which to define it (i.e. Good). Which Form, is the only good and righteous being in existence: God. Furthermore, as that God has revealed himself in His Word (cf 2 Timothy 3:16; Hebrews 1:1) and Law (Exodus 20), then sinful man can only define morality based upon the standard of God's Holiness which is the perfect "Form" for all moral understanding.
Next Dr. Zak says:
"We then found oxytocin was responsible for many other moral behaviors, from being generous to sacrificing to help a stranger."
The latter part of this statement is intriguing as Dr. Zak mentions in his video presentation stress and testosterone decreased oxytocin levels, and that chemical is directly related to feelings of trustworthiness. Zak determined this by a test that he outlines in the aforementioned article and video, but there is something to address that is more pressing. Trustworthiness aside, Dr. Zak also said that a decrease in oxytocin levels (peripheral, not central; more information on oxytocin is available here) leads to selfishness. Finally, Dr. Anne Campbell who wrote the article "Oxytocin and Social Behavior," stated that oxytocin release happens only after stimulus (e.g. touching, massage, prayer are all examples Dr. Zak used; Dr. Campbell also cites childbirth). Only one question then remains: What about combat and the sacrifice of one soldier to save his comrade (high stress, no stimulus) or a civilian for that matter? What about a man or woman who braves a burning building or raging river to save a stranger's life (again, reactionary and w/o stimulus)? Dr. Zak has presented the conclusion to his research as definitive proof that a chemical makes someone "moral," yet he fails in the point just presented. Morality is not chemical, it is metaphysical and relies on God for its very definition and existence. The only way one can achieve a saving "goodness" is by the covering of that person by Christ's righteous sacrifice on the cross (Colossians 2:13-15).
One final thought, all of the responses that related in increased peripheral oxytocin in Dr. Zak's study were empathetic to another party and involved money, and the majority of research about oxytocin in the first place is related to rodents...just saying...