Humans face moral dilemmas, from fleeting everyday decisions to life-or-death questions.
Should I give up my seat on the train for a frail, elderly person? Do I snatch a cab from another who has been waiting longer just because I have a pressing matter to attend to?
Should I have an affair?
If there are two similar, competing organ recipients, which one deserves that second shot at life more?
Morality is a murky soup of nature versus nurture, it seems.
Some scientists believe that our moral compass is hardwired into our brains while others believe that our surroundings and people close to us help shape our thoughts and actions.
While the experts have their disagreements on this subject, they concur on this: The tussle between doing good or evil resides in the brain.
Dr Melanie Storry Chan, a consultant psychologist at The Counselling Place, said: 'Moral decisions reside in the ventromedial prefrontal cortex (VMFC).
'This area of the brain is responsible for highly complex functioning such as empathy and the ability to make moral judgments.'
The VMFC in an adult is the size of a large plum and is found about 5cm behind a person's forehead.
The case of Phineas Gage, a railway construction foreman in Vermont in the United States, famously illustrates how the VMFC affects one's moral bearings.
In an accident on Sept 13, 1848, a steel rod entered the 25-year-old's left cheek bone and exited through the top of his head.
Although he later recovered, his personality was forever altered as most of the frontal cortex of his brain was destroyed.
Dr Storry Chan said: 'Before the brain injury, Gage was considered a good worker, popular and efficient.
'But after that, he was angry, used the grossest profanities, became impatient and was 'no longer Gage'.'
Indeed, a 2007 American study of patients with ventromedial injuries and those without such injuries found that the VMFC is key to making a moral call.
Dr James Khoo, a consultant neurosurgeon at Neurological Surgery, gave the example of a brain that is damaged by dementia.
'I see a change in people who develop dementia. They swear more, may steal things or do things they previously knew were bad,' he said.
The ventromedial area is connected to other regions like the brain stem, responsible for bodily functions like breathing and heart rate; and the amygdala, important for perceiving and reacting to emotional or affective behaviour and feelings like fear and anger.
It thus acts as a central system, collating information from these various brain regions to come up with an appropriate social response. Damage to this area disrupts or severs this vital function.
Another major area also comes into play in moral decisions - the dorsolateral prefrontal cortex which approaches situations in a strictly cost-benefit analysis way.
A study last year by the University of Illinois, Champaign-Urbana and the California Institute of Technology found that our moral decisions are also affected by how much weight our brain places on fairness.
Just last month, a study published in the scientific journal PNAS found that individuals who behave dishonestly exhibit increased activity in parts of the prefrontal cortex, whether they chose to behave dishonestly or not.
So, it looks like issues of morality are parsed and processed in the brain. However, researchers do not know if the parameters within which the brain works are a matter of biology or the environment.
Dr Khoo believes that it is a canny mix of evolutionary biology and socio-environmental factors.
He said: 'Most humans have a sense of what's right or wrong. From an evolutionary perspective, what's good for the tribe is good. We act on what we think protects our group.
'But how we've been brought up, how we think, parental teaching, religion and all that come into play.'
Indeed, most children are able to differentiate between right and wrong by the time they go to pre-school or nursery school.
Moral behaviour is often learnt first via concrete rules set by authority figures like parents or teachers, and then by understanding abstract concepts about morality.
Dr Storry Chan said: 'We tend to first tell the child something like 'It is not nice and against the rules to take your friend's toys. I will punish you if you do that again'.
'As the child grows older and begins to think more abstractly, we can tell the child 'Your friend will be angry if you take his things and you will feel bad for doing it'.'
It has been observed that, in making moral decisions, boys focus on 'justice' while girls tend to be more concerned about maintaining relationships and will change the activity rather than argue or fight.
Dr Khoo suggested that exposure to morally upright people can help strengthen the brain mechanisms which deal with moral decisions.
He said: 'I don't think people are born to do good or bad. They're influenced by their early childhood and own ways of thinking.
'Mother Teresa probably had poor but good parents. She was exposed to religion in her early life so she decided to dedicate her life to doing good. People do stumble on such things in an experiential way.'
The late Mother Teresa was born in Albania in 1910 to Nikola and Drane Bojaxhiu. Her father Nikola died when she was eight, leaving the family financially poor but Drane raised the children lovingly, influencing Mother Teresa's character and later vocation.
How willing are we to help others? We seem to be more compelled to help those around us.
Dr Storry Chan explained it thus: 'Choosing to help people near us is a function of many factors, for example, cultural, governmental and proximity. It is easier to remember to help someone you see every day rather than someone you see on TV once a fortnight.
'Needy people close to us are able to 'push our emotional buttons' much more effectively than those far away, leading to our greater desire to help those closest.'
As for the 'pay-offs' in doing good, these come in positive things like an increased sense of satisfaction with life, higher levels of self-esteem and a slight reduction of heart rate and blood pressure.
The converse can happen if we choose to do something bad.
People who have done wrong things often report feelings of low self-worth, guilt and anxiety. They may get symptoms like difficulty in sleeping, nervousness or heightened feelings of depression and anxiety.
Dr Storry Chan said: 'However, the impact on the person after choosing to do something 'bad' depends on how much the behaviour deviates from his own moral values.
'For example, a person who does something considered amoral or completely against his morals will have a higher rate of negative experiences than if he does something he can justify or excuse.'
She added that guilt is a healthy, natural emotion which reflects a conflict between our actions and our moral compass.
Asked how a person who has committed a bad deed can address his guilt, Dr Storry Chan said: 'You would look at what's the reason for the behaviour, the damage done.
'Also, whether there are avenues for restitution or apologies or penance, alternative choices if put in similar situations in the future, seeking forgiveness from others and then seeking self-forgiveness.'
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