Neil Patrick Harris revealed why he doesn’t want to know which twin is biologically his as fans left divided

Neil Patrick Harris is every inch the proud dad of his twin children, Gideon and Harper Grace, and despite being the biological father to only one of them, he isn't interesting in finding out which one.

How I Met Your Mother star Harris and husband David Burtka welcomed the twins via surrogate back in 2010.

The children were conceived using an anonymous egg donor, as well as sperm from both Harris and Burtka.

Each of them is biological dad to one of the twins, but they chose not to know which one is biologically theirs.

The family appeared on the red carpet this week for the premiere of The Lost Boys on Broadway, and fans couldn't believe how grown up the 15-year-old twins looked.

While some fans were mourning the passage of time and saying sadly that they felt old seeing how much the children had grown up, others were comparing the twins to their dads and guessing who was the father for each.

The family appeared on the red carpet and fans have been guessing who looks most like who (Photo by John Nacion/FilmMagic)

"I've been trying to guess which twin looks like which father, but I can't tell," wrote one, as another joked: "The boy looks so much like the one who isn't Neil Patrick Harris."

"David's nose is literally copy and pasted onto the boy!" insisted another, while this commenter agreed: "Definitely ! The boy looks like David and the girl like Neil."

"The son looks exactly like NPH’s husband. And their daughter looks like NPH," mused one viewer, while another added confidently: "Yeah I would bet money that NPH is the bio father of Harper and David is the bio father of Gideon. seems fairly obvious to me."

Despite the observations of fans, nobody knows the true answer for sure.

Back in 2014, Harris spoke to American broadcaster and journalist Barbara Walters about why the couple made the decision they did.

The twins were welcomed by surrogate back in 2010 (Photo by Jon Kopaloff/FilmMagic)

He insisted that neither he or Burtka have any interest in finding out who of Harper or Gideon is biologically his.

He said: "We inserted one of my sperm and one of David's sperm into two eggs with the hope that they would both take, just because we both wanted to be dads biologically. Both took, miraculously."

He insisted: "I have no interest in [finding out]. We are their parents and I love them implicitly."

Harris added that as the twins have grown up, he and his husband have an inkling which way round it might be 'based on their behaviour'.

Fans always look forward to the Halloween and Christmas pictures that the couple share every year, and always debate over who looks the most alike, but it seems we'll never know for sure.