When Dr Cai Yiming was 13, the Malaysian town of Teluk Anson was gripped by a sensational murder case.

In 1962, a businessman allegedly kidnapped and later drowned a rival in a mining pool. After he was arrested, he led the police to the scene of the crime. A frogman died while trying to retrieve the body.

The ensuing trial attracted hordes, including Dr Cai, now a 68-year-old psychiatrist, who would rush to court every day after classes.

Because DNA testing was not available in those days, the businessman's lawyer managed to get him off. The court ruled that there was not enough evidence because the body of the victim was too badly decomposed to be identifiable.

Like many people, Dr Cai's late grandfather was outraged by the outcome. He told his grandson that he should always stand on the side of justice and work to improve the lives of others when he grew up.

The words stuck.

Today he is one of Singapore's most respected experts in two areas: child/adolescent psychiatry and forensic psychiatry, and is often called upon to give his expert opinion in high-profile criminal cases.

Among others, he conducted the psychiatric assessments of the 15-year-old student who was enticed with $100,000 by Anthony Ler to kill his wife in the infamous 2001 murder case.

Chatty and humorous, the good doctor says what he is doing now is not the result of meticulous planning.

"I've never believed in planning too far ahead; such plans will never come true anyway. At every stage, I just go along with where life takes me," he says.

And life has taken him on quite a journey: from a farm boy in Bukit Pagar in Tapah, Perak, where he remains the only doctor the 70-year-old village has produced; to the most experienced child as well as forensic psychiatrist at the Institute of Mental Health (IMH).

Bukit Pagar, still home to 2,400 people, is one of Malaysia's ubiquitous "new villages", originally built by the British in 1947 to segregate villagers from communists.

"As a kid, I remember having to collect rice cooked in a big communal pot. We couldn't cook rice at home because they were scared we would supply it to the communists," recalls the eldest of nine children.

His parents, farmers with little education, had seven daughters before another son came along. One of the daughters was given away.

His childhood days were halcyonic.

"I helped to bathe the pigs and chop up yam leaves to make swill for them. But I also spent a lot of time climbing hills (and) catching catfish and butterflies," he recalls.

One of only two A pupils in his village Chinese primary school, he had wanted to continue his secondary education at Penang's Chung Ling High School, which he described as "one of the best Chinese schools in South-east Asia".

When that plan was dashed because his father could not afford it, he continued his education in English at secondary schools first in Tapah, and later in Ipoh.

Dr Cai's life could have turned out differently if his father had not intervened a second time.

His results for Malaysia's Lower Certificate of Education - taken when he was in Secondary 3 - were so good it qualified him for a scholarship from the Royal Military College in Kuala Lumpur.

But his father, who took part in the anti-Japanese movement during World War II, was dead against the idea of military college.

"He said: 'Are you crazy? Do you know what war is like?'"

So Dr Cai completed the Malaysian equivalents of the O and A levels at the Anglo-Chinese School in Ipoh instead.

Ipoh in the 1960s, he says, was a vibrant town because of the booming tin industry.

"All the towkays (business owners) had more than one wife, one house and one Mercedes," says Dr Cai, who rented a room from the mistress of a businessman.

"Many of them didn't do well in their studies but were so rich. I told myself that I would also one day drive a Mercedes," adds the owner of a Mercedes E200 with a laugh.

His sterling results got him into medical school at the University of Singapore. Asked if it was a dream he harboured, he replies with a laugh: "No, my results were good so I just went with the flow."

After graduating in 1975, he did his rounds as a medical officer in a couple of hospitals before he was posted to mental hospital Woodbridge in 1977.

Woodbridge, now part of the IMH, was like a no man's land in those days.

"There were 3,000 beds but only 15 doctors. A lot of people didn't want to go there because there was a stigma about mental illness. People thought it was contagious," he says.

"But I was fascinated. Mental illness is sometimes so incomprehensible. Every case has a human story and involves emotions, family and society."

He acquitted himself well, and so was sent on a two-year scholarship to get his specialist diploma in psychological medicine from the Institute of Psychiatry in London.

On his return in 1981, he worked at IMH treating patients for mental conditions - from bipolar disorder to depression and obsessive compulsive disorder.

Seven years later, he was assigned to the prison to do psychiatric evaluations of hardened criminals. The majority of the cases he handled involved murderers or drug traffickers.

His job was to determine if they had any mental condition leading to an unsoundness of mind or diminished responsibility, which meant they should be spared from the noose.

"One of those I assessed was a murderer who was from my primary school. He killed his girlfriend," he says.

A year later, Dr Cai moved into child and adolescent psychiatry.

"I wanted a change in environment and do something new," says Dr Cai, who is the first child psychiatrist to be given the title of emeritus consultant.

He spent a year based in the Hospital for Sick Children in Toronto as a clinical fellow and, for more than 10 years until 2006, was the head of the department of child and adolescent psychiatry in IMH.

"I did everything: clinical assessment, treatment, education of doctors, public education as well as research," says the psychiatrist who was also a World Health Organisation consultant for child mental health in Beijing.

The problems afflicting children and teens in Singapore are mostly psychosocial in nature, he says.

"The feeling of being neglected, not loved. The emphasis (on) academics and the pressures which have become more intense. Bewilderment because of parental discord.

"Harsh discipline, communication problems, rivalry in school," he continues.

The result? Developmental, behavioural and emotional problems.

"Obsessive compulsive disorders, violence, stealing, unwillingness to pay attention or learn," says the author of several books on child psychiatry, including Living With ADHD (2003) and When Parents Fight, The Children Cry (2005); as well as research papers published in local and international journals.

He is often called upon to clinically assess teens in high-profile cases like that of the 15-year-old student. Mired in debt, Ler plotted the killing of Annie Leong - who wanted a divorce - so that he could get their maisonette. He coached the teen on how to kill her.

Shaking his head, Dr Cai recalls: "The boy was promised $2,000 a month for more than four years. That, to him, was a lot of money. He was influenced purely by temptation."

Ler was hanged in 2002. Because of his age, the boy was spared the death penalty and detained indefinitely.

Another teen whom he assessed was Amos Yee, charged with and convicted for engaging in hate speech in 2015 and last year. The 18-year-old is now detained in the United States awaiting a decision on his asylum application.

Of the teen, the psychiatrist would only say: "He forgot that freedom (of) expression is not freedom from responsibility."

Dr Cai has a vault of interesting anecdotes.

He once worked with other experts, including the late Chao Tzee Cheng - one of Singapore's best-known forensic pathologists - to crack the case of a four-year-old boy who survived a fall from the ninth storey of an Housing Board block in 1995. The family's domestic worker who fell with him died.

To interview the boy, who could not describe the incident clearly, Dr Cai and team used props including a toy house to set up a play situation. It turned out that the maid had tried to hold back the boy who was trying to retrieve a ball he had kicked out of the window. She held him from the back before they fell.

With a smile, Dr Cai says what he does can be simple, or extremely complex.

"Sometimes all it needs is a transfer to another school to help a child with behavioural problems. But at other times, despite trying to the best of my abilities, I need to look for other resources or refer them to other agencies.

"And some cases deserve to be locked up because they are a threat to society," says Dr Cai, who is married to his secondary school sweetheart, a former teacher. The couple have two children in their 30s; one is a psychiatrist, the other is an IT professional.

Asked what advice he would give parents in Singapore, he says simply: "Don't overpamper or be overly worried. Give your children space to develop and learn from their mistakes." Do not push them to "overlearn", he adds.

"It's not helpful. It's more important to impart good basic values so that they can learn to solve their own problems."

Dr Cai Yiming recently won the National Healthcare Group (NHG) Awards 2017 for his outstanding contributions to public health.