It's something no parent wants to admit, but according to an Australian study, 69 percent of parents can identify a favourite child, and 80 percent could identify a family 'black sheep'.
If that research is to be believed, it means almost seven of ten parents reading this are probably feeling pretty guilty right now.
But what does that mean? Does it make you a bad parent? Or is it merely part and parcel of raising a group of little individuals, with their own personalities and interests and quirks?
"There's a lot of guilt surrounding this issue as a parent, but it's funny, everyone can go 'oh yeah that's me' if they relate to it through their own sibling. They'll say, 'no wonder my brother got the easy run'," parenting author, educator, and director of Parenting Ideas Michael Grose told HuffPost Australia.
"Often from my experience [favouritism] shows itself through discipline or management, so a parent will be tough on one and not on the other.
"The other interesting thing is, even kids know about the favourites themselves. You'll often hear things in families where the child will recognise this. One child will go 'you go and ask mum. She'll listen to you'."
Why you have a favourite child
"I think there's this notion that families are a nice and level playing field but really, there are a lot of complexities that play within," Grose said. "In a way families are just big groups except we're all related to each other. Every parent loves their kids, but they don't like them all the same. And there are reasons for that.
"I think some of the reasons are around some personality types being easier to relate to than others. So if you're an achievement orientated person and has a child who achieves and is an ambitious kid, that's an easy kid to like.
"Also the other aspect with our children is that we can recognise aspects of ourselves in them, and it makes easy to relate to them."
"For example, I'm a worrier myself and I have a daughter who's a worrier and I really relate to that. My wife doesn't get it because she's so easy going and she just throws her hands up and says 'what's she on about?' Whereas I'm like, 'I get it. I'd be worried too'."
Grose also says gender can play a role, depending on the family make up.
"Youngest girls always have their father wrapped around their little finger," he said. "Dads are often drawn to their youngest girls very strongly.
"Dads will often drive their older son to achieve, so he can be an absolute favourite if he is achieving. They may have a younger son who is similar but he probably won't be as drawn."
Mums aren't let off the hook either, with Grose noting many mums favour their firstborn son, even if it's just a way to protect them against the father's tendency to drive them to succeed.
"This gets complex, but as I said before, dads will often drive their firstborn sons, and mums will be drawn to them as a way to nurture and protect," he said.
However this isn't necessarily a bad thing.
"Functioning families do that as well," Grose said. "Parents who get on well and function well, I think play good cop/bad cop. And so if one parent is a bit tougher on the other, sometimes the other parent will go, 'oh come on lighten up, you can have some ice cream'.
"There's nothing too much wrong with that. What we do know is the best atmosphere for raising kids is they need firmness and boundaries as well as nurture and encouragement. Often a child will get one from one parent and the other from the other."
Is it normal?
So we've established that lots of parents have a favourite child, and, perhaps worryingly, the child can even sense it. But what is 'normal' favouritism and what isn't?
"You need to to be careful about having one rule for one child and one rule for another," Grose said. "Now, I don't want to be black and white about this because if you have child with special needs the situation is obviously very different.
The business of having one rule for one child and another for the other is completely different. Kids can resent that.
"But it's also human nature. When it comes to understanding and forgiving, we are always stricter with our firstborns. That's the bar we'll use and then we'll relax more as we move our way through the family.
"So, for instance, most of the time the youngest one will be allowed to go to parties under the age than the firstborn [was allowed to]. And that's okay.
"But the business of having one rule for one child and another for the other is completely different. Kids can resent that."
How to manage it
"Stick by the rules," Grose said. "Families work really well if you recognise the hierarchy that's within them.
"For example, older kids have certain responsibilities but they do have extra privileges, like a bit more pocket money or going to bed later.
"In saying that, there can be resentment if all the responsibility lands on the older one and not the young one. My advice is to spread the load as well as the love across the family."
Grose also says children should have to wait for certain privileges, a premise that smaller families tend to struggle with more than a family of four children or more.
"If you have two children or less, parenting is an individual endeavour," Grose said. "If you have four or more, you're the parent to a gang.
"In large families, the rules tend to be more concrete. 'You get your big bike when you're seven'. That kind of thing, it's almost pinned. Kids know that. They learn to wait.
"In a small family, what we often find with parents is they say, 'you can have a bike, you're seven. Oh no, our poor five year old feels left out, you can have one too'."
And while there's nothing like the love of a parent, Grose also says it's really important for kids to interact with their extended family. Middle children in particular could benefit from this.
"Having an aunt or grandparent or family friend can be a godsend for kids who feel left out," Grose said.
"If you've got three, maybe the one in the middle feels left out, and sometimes it could take another person outside the family to see something a bit special in the child.
"We often are drawn to the things we like in kids, and that all works well if your kid has the thing that you are drawn to. So if mum and dad are both sporty, but have a child that is more artistic or has a left field bent, they can can be harder to relate to."