Parents really do have a favourite child.

The favourite child. It’s a gag that crops up in almost every family – in fact, two days ago, my mother opened my birthday card with the following line:

To my favourite daughter… 

I’m her only daughter.

It may be a cultural joke but a long-term study by sociologist Katherine Conger suggests there really is a favourite child and news flash, it might not be you.

The study, conducted in America, followed 384 families where a pair of siblings were born within four years of each other.

Results revealed that 74 percent of mothers and 70 per cent of fathers reported preferential treatment towards one child, according to Quartz.

The study also found that first-born children were more likely to believe they are the preferred child and younger children are more likely to believe the elder children are too.

So according to Conger, I think I'm the favourite and if I had a younger sister, she'd think I was too.

Quartz also spoke with Conger about the study where she revealed one of her main theories had been proven wrong.

“Our working hypothesis was that older, earlier born children would be more affected by perceptions of differential treatment due to their status as older child—more power due to age and size, more time with parents— in the family,” Conger said.

Conger also told Quartz there exists a belief within every sibling they're being treated unfairly.

“Everyone feels their brother or sister is getting a better deal,” Conger said.

“Regardless of how you look at it, both [elder and younger children] are perceiving preferential treatment.”

The concept of the 'favourite child' has been echoed by other medical professionals such as developmental behavioural paediatrician Dr Barbara Howard.

Dr Howard told the New York Times she often saw children develop behavioural issues from not being seen as the preferred child.

“It’s impossible not to have favorites, and we do know that the perception of favoritism is one of the biggest factors in sibling rivalry,” she said.

Here's one mum who openly talks about having a favourite: