There are multiple studies that show that
morality has nothing to do with a belief in God. You can have, and follow moral precepts, any religion. An atheist can believe in the need to respect human life, just as a believer can believe in free will and individual rights.
You may be interested in the work of Lawrence Kohlberg. He was a physcology professor who did a lot of research into how people reason about what is moral.
Kohlberg came up with five stages of moral reasoning. These stages do not determine who is more moral, but rather determine the complexity of the reasoning you base your moral decisions on.
For example a young child in the first stage bases his decisions on whether he will be punished. A older child makes decisions based on relationships (if it makes mommy happy, its good). An even more advance person understands rules and laws. To Kohlberg the highest level of moral reasoning is based on an abstract understanding of deep moral principles (supposively few of us attain this level).
An interesting part of this theory is how we develop morals. You will stay at a level of moral reasoning as long as it works for you. When your level of moral reasoning is challenged-- meaning that it can't give a satisfactory answer to a problem you face, or that it leads to an inherent contradiction-- it will lead to a period of confusion followed by a resolution at a more advanced level of reasoning.
For example, an infant thinks it is entitled to whatever it wants, but it will be "moral" to avoid punishment. When the infant begins to develop a more mature relationship with other people, it realizes that if everyone thought this way, it would lead to chaos. It is clear that Mom is acting "morally" to take care of baby, but no one is punishing Mom. This pushes the baby to understand "relationships" and reach the point of "I do good to Mom because she does good to me".
In this theory the third stage is "conformity" meaning it is moral to do what everyone else does (or what people expect). The fourth stage is "law and order" meaning (as it sounds) you live according to the rules.
Many adults live most of their entire adult lives in one of these two stages. It is certainly possible to do what people around you expect and obey the law and never face a dillema that this level of thinking won't face.
According to this theory,
Moral Relativity comes about as a result of dilemmas that the fourth stage (following the law) can't solve. Kohlberg acutally discusses an "in-between stage" between 4 and 5 which he saw in his research. This is often a time of confusion and is common in very smart teenagers.
The theory says this is resolved with the fifth stage which is "social contract". A person in this stage realizes that they need to adopt a set of morals (which may not be her natural morals) in order to live in a society.
The sixth stage is following Universal morals. Since few reach this stage, it isn't worth discussing.
There is a seventh stage that some people talk about (I don't think it is from Kohlbergs work). It sounds pretty goofy to me.
link: Page on Kohlbergs Stages