At least three quarters of the human genome consists of non-functional, 'junk DNA', according to a new study, and the actual proportion is likely to be even greater than that.

Ever since Watson and Crick discovered the double helix structure of DNA back in the 1950s, scientists have been debating what extent of the genome is responsible for making you you – and now an evolutionary biologist says the answer to the riddle lies in some basic math.

Dan Graur from the University of Houston calculates that the functional portion of the human genome probably constitutes only about 10 to 15 percent of our overall DNA, with an upper limit of 25 percent.

The rest of our genome – somewhere between around 75 to 90 percent of our DNA – is what's called junk DNA: not necessarily harmful or toxic genetic matter, but useless, garbled nucleotide sequences that aren't functional in terms of encoding proteins that spur all the important chemical reactions going off inside our bodies.

The rationale for Graur's model is based on the way mutations creep into DNA, and how as a species we weed these mutations out for the benefit of all.

These kinds of genetic variants, called deleterious mutations, appear in our genome over time, subtly shifting or reordering the four chemical bases that make up DNA – adenine, cytosine, guanine and thymine – in parts of our genetic code.

When mutations take place in junk DNA, they're considered neutral – since that genetic code doesn't do anything, anyway – but when mutations occur to our functional, defining DNA, they can often be harmful and even ultimately lethal, as they mess up the instructions that code for healthy tissue and biological processes.

On that basis, it's better for our evolutionary prospects if less of our DNA is functional, because less of it is then exposed to the risk of mutation and the increased chances of early death it invites.

In Graur's calculations, given the risk of deleterious mutations to the survival of the species on one hand – and the known stability of population and reproduction rates throughout human history on the other – the limit of functional DNA has to be very low.

Otherwise dangerous mutations would keep stacking up, meaning we'd have to produce impossibly huge numbers of offspring for the small percentage of healthy bubs to survive.

"Under the assumption of 100 percent functionality and the range of deleterious mutation rates used in this paper, maintaining a constant population size would necessitate that each couple on average produce a minimum of 24 and a maximum of 5 × 1053 children," he writes in his paper.

Of course, nobody really other than creationists is suggesting that we carry around zero junk DNA, but a huge 2012 study called the Encyclopaedia of DNA Elements (ENCODE) project did claim that as much as 80 percent of human DNA was functional.

That study was controversial – partly because many scientists claimed that the ENCODE definition of 'functional' was too broad.

In Graur's use of the term – where functional DNA is code that's evolved to be useful in terms of its evolutionary effects – the 80 percent figure just doesn't add up.

"For 80 percent of the human genome to be functional, each couple in the world would have to beget on average 15 children and all but two would have to die or fail to reproduce," he writes.

It's more likely then that only about 10 to 25 percent isn't junk DNA, Graur thinks.

While his is unlikely to be the last word on the subject – the new results do coincide somewhat neatly with the findings of a separate 2014 study – and could help focus vital scientific efforts on researching a smaller window of uncontested, 'functional' DNA.

"We need to know the functional fraction of the human genome in order to focus biomedical research on the parts that can be used to prevent and cure disease," Graur says.

"There is no need to sequence everything under the sun. We need only to sequence the sections we know are functional."

The findings are reported in Genome Biology and Evolution.