Singapore - A few of us were at our friend S's place when he grabbed one of his twin boys in a bear hug and planted a kiss on his head. Then, realising it was the older boy, he said playfully: "Oops, wrong one."

Scandalised, we stared at him.

His wife had long told us that the younger twin was his darling, but we were surprised he made no attempt to disguise his feelings in front of the pre-schoolers.

The topic of favouritism is a contentious one: Most people believe it exists, but most parents will flat out deny it.

Two years ago, American journalist Jeffrey Kluger stoked the debate - and no doubt tore open scores of old wounds - with a cover story for Time magazine titled Why Mom Liked You Best: The Science Of Favoritism.

"It's one of the worst-kept secrets of family life that all parents have a preferred son or daughter," he wrote.

He cited as evidence a 2005 study by researchers from the University of California, Davis, who followed about 400 pairs of siblings and their parents for three years, and concluded that 65 per cent of the mothers and 70 per cent of the fathers were partial to one child.

Mr Kluger did not explain how 65 or 70 per cent worked out to be "all" parents. But in his 2011 book, The Sibling Effect: What Bonds Among Brothers And Sisters Reveal About Us, the father of two reiterated that "95 per cent of parents in the world have a favourite child - and the other 5 per cent are lying".

I have been using this as ammunition against my poor husband, even as I pooh-pooh the second half of the exaggerated statement and insist I am scrupulously fair.

I sometimes accuse him of favouring our three-year-old daughter over her older brother, a charge he denies just as vigorously. My iron-clad proof? He speaks to her in gentler tones and is often harsher when it comes to disciplining our boy, who is six. Even the punishment he doled out to the boy for similar offences our son committed at her age was invariably heavier, I maintain.

Children who are too young to fully grasp and articulate the concept of favouritism can sense it too.

"Mei mei is very cute, right?" my son once asked my mum, not long after his sister was born. When she agreed, he concluded, after processing all the fuss the adults were making over her: "Cuter than me, right? I'm not cute."

Still soft and cuddly, his sister sometimes draws the admiring coos adults reserve for cherubic tots. And this exacerbates the feelings of insecurity he has had since she came along.

It is a vicious circle: He sees her as a supplanter, so he fights her for everything - toys, food and, of course, attention. The adults are often forced to go to her defence because, as the younger one, she is deemed more vulnerable.

This confirms his suspicions that he is playing second fiddle. So, ignoring our reasoning and reassurances, the aggressive behaviour continues and he usually ends up being punished because of her.

Last week, after she reported yet another alleged misdeed of his, she added: "Mama, that is not right, hor? I don't do that."

Her brother gave her the evil eye and snapped, finally spelling out his frustrations: "Stop it! You just want Mama to say you are a good girl and I am the bad guy."

But my boy, I wanted to tell him, you are precious to me no matter what.

I will always have a soft spot for him simply because he's my first-born.

He was, and still is, the source of countless new and scary emotions and experiences that will forever be seared in my mind. Because of him, I discovered strengths - and flaws - I never knew I had. He was the little guy whose arrival upended our lives that now, strangely, feel just right.

Columnist Francesca Kaplan Grossman, a mother of two, put it beautifully in an article for The New York Times last month: "I love my first with the love of firsts. The breathtaking vastness of Cape Cod beach in the very early morning. The first day of camp, friendless and waiting... I love my second from a whole different angle - from the soft place of experience. The golden beach at 4pm. A lifetime of friendship with the girl with the long blond braid."

Indeed, I have warmer memories of my daughter's first year because she gave me the chance to redeem myself. I learnt to put right several things I struggled with as a first-time mum and got to enjoy her properly.

Perhaps my growing confidence rubbed off on her. Perhaps she was born with a sunnier disposition. Whatever it is, my girl has been the easier of the two so far and that endears her to the grown-ups around her.

Does that mean we love her more? No, it just means we deal with her differently than we do her brother. And the dynamics will keep morphing as they grow. As with all human relationships, our ties with our kids are complex and nuanced, waxing and waning over time, depending on countless variables from age and gender to chemistry and personality.

I gained fresh insights when I broached the subject again recently with our friend S. It wasn't favouritism but profound sympathy that he was showing towards his younger son, he said.

His wife had suffered complications during their birth, which had a more serious health impact on the younger twin. Six years on, he is bright and perky, but remains on medication and is noticeably shorter than his brother.

"I feel helpless when I look at him. He reminds me of what I need to do to make his life happier, like a longer hug," S explained. "All the money in the world is not going to bring him back to 100 per cent."

He may appear more affectionate towards the younger boy, but he is equally protective of both sons. "As a father, I would do anything for either of them."

At the end of the day, I reckon that is what counts. As Ms Grossman put it, it doesn't matter if we love our kids in different ways, just as long as we love them equally much.

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