Conscience tends to be defined as the feeling that may make a person believe that certain actions, or failures of actions, are inherently wrong. When a person ignores these feelings, he or she typically feels guilt or remorse. Philosophers, religious leaders, psychologists and a variety of others have tried to determine the source of such emotions, and many arrive at different answers.
In many religions that worship the Judeo/Christian/Islamic god, conscience is a God-given facility; something that people have with us from birth. It’s still up to parents to help shape a child's behavior by teaching about what is "right and wrong." Catholics define the age of seven as the Age of Reason, where a child has the facility to understand this difference, and knows enough about sin so as not to commit it.
Even though consciences are God-given from many religious viewpoints, very young children seldom seem to listen to theirs. A child who wants another child’s toy may simply take the toy. Parenting helps the child understand that people should not just take what they want. This may take some children a few years to learn.
Many people talk about the "little voice in the back of the head" that makes them feel with extraordinary conviction that an action committed or not committed may be wrong. The little voice may nag, complain, or make a person feel harassed when he or she acts in a way that might not be considered right. This leads quite naturally to early psychological explanation of consciences.
To psychologists like Sigmund Freud, the voice within was the superego, the set of rigid moral precepts that helped to control the want and take aspects of the id. Superego is the sum of things learned early in life about right and wrong, which impose themselves on the id to allow the self or ego to function within the bounds of a particular society. A person with poor superego control is considered id-driven.
Some believe that this feeling is best described as the voice of reason. This was the view of Thomas Aquinas and many other philosophers, although they may differ on the source. Reason is God-given to Aquinas, and therefore consciences are God-given. Humans have a capacity to develop reason, however, and don’t necessarily do; so reason remains a facility that must be exercised and developed. To act conscientiously is to act on reason-based decisions.
An interesting forward move occurs when Aquinas discusses people who have erring consciences and make faulty decisions based on faulty reasoning. He states that this may not be the person’s fault if he has not learned enough to know what is right or wrong. Such an idea might be applied to the modern sociopath, who is said to act without having any emotional connection to right and wrong. Perhaps the sociopath had the ability to apply reason thwarted at an early age.
This idea that right from wrong must be taught occurs in many philosophical, secular, and religious theories. Learning reason, the moral code of society, or what constitutes right and wrong lead people to that "inner voice" that tells them when they are about to make a mistake. This may be considered intuitive, especially if someone used to hearing that voice, but it has become intuitive through a set of learned behaviors.
Conversely, if conscience is viewed as an inborn, fully developed trait, a person already has an implanted moral code and truly does come into the world with a sense of morality. Whether this is considered from a religious perspective or from an anthropological and social perspective, this sense may be one of the remaining animal instincts, geared toward human survival and maintenance of societal structure. The "inborn" theory doesn't account well for people who seem born without consciences, the so-called "bad seeds" of society.
More and more though, such bad seeds, especially those kids who weren't abused or parented poorly, are considered ill rather than evil and seem to have missed a crucial inborn instinct — perhaps due to genetic mishap — that would give them a moral code. What is unclear is the religious perspective on this mentality.