How older people feel about younger people says far more about themselves than future generations.

At some point in our lives, we've all heard or grumbled about "kids these days". It's a popular sentiment that's been felt for millennia, and as it turns out, it's probably not the youth's fault, nor has it ever been.

A major review of five studies suggests this timeless notion is a mere psychological illusion - a subtle mental 'tic' so deeply-embedded most of us aren't even aware it exists. And it's been named "the kids these days effect".

"There is a psychological or mental trick that happens that makes it appear to each generation that the subsequent generations are objectively in decline, even though they're not," says research psychologist John Protzko from the University of California Santa Barbara.

"And because it's built into the way the mind works, each generation experiences it over and over again."

Our biased memories and opinions of ourselves appear to underpin this fault. Drawing on responses from 1,824 American adults, Protzko and his colleague Jonathan Schooler first looked at whether stricter adults are more likely to see kids as disrespectful.

Their second study then examined whether smarter elders are more likely to feel kids are less intelligent. And their third study, among 1,500 different adults, explored whether well-read people consider today's children unusually book-averse.

In every single test, the higher a participant ranked their own traits, the more likely they were to denigrate those same characteristics in today's children.

"The more you respect authority as an adult, the more you think kids no longer respect their elders; the smarter you are, the more you think kids these days are getting dumber," explains Protzko.

"And people who are well-read tend to think that kids these days no longer like to read."

To explore what could be driving these beliefs, the authors conducted two more studies. The first follow-up replicated study three, but this time it included questions about how much people enjoy reading, how much they enjoyed reading as a kid, and how much they recall their childhood friends enjoying reading.

The findings reveal participants who consider themselves well-read tend to feel the exact opposite about others nowadays.

"This finding supports the conjecture that people who are objectively elevated in a trait are particularly predisposed to notice others (both youth and adults) as lacking in that trait," the authors explain.

"While people may believe in a general decline," they add, "they also believe that children are especially deficient on the traits in which they happen to excel."

This suggests that while people do not think each generation is becoming steadily less interested in reading, they appear to regard today's children as uniquely deficient, more so than any before them.

This attitude, the authors argue, stems from how a person falsely "remembers" their childhood. Those participants who said they liked to read in their youth were also more likely to remember their friends enjoying books, potentially skewing their whole concept of reading enjoyment at the time.

In the fifth and final study, Protzko and Schooler took 1,500 new participant's and manipulated the way in which they regarded their own reading abilities. Then, they tested once again how these thoughts translate to opinions about today's youth.

"We told some of them they were in the top 33 [percent] of the national population or in the bottom 33 [percent] of the population," Protzko explains.

"It turns out that doing that changes how they feel about their own standing about being well-read."

In other words, by altering a person's recollection of the past, the authors were able to soften attitudes towards the younger generation, at least in respect to reading enjoyment.

Looking back over the results of all five studies, they conclude that the "kids these days effect" is probably a subconscious human tendency, wrapped up in our biased memories of ourselves and our peers.

"It's a memory tic - you take what you presently are and you impose that on your memories," explains Protzko.

"It's why the 'decline' seems so obvious to us. We have little objective evidence about what children were like, and certainly no personal objective evidence. All we have is our memory to rely on, and the biases that come with it."

Natasha Rajah, a psychologist at McGill University who was not involved in the study, told Popular Science the reality is probably more complicated. She says the traits chosen in this study were generally valued more by older people - unlike the use of technology, which today's youth tends to excel at.

"Is this truly thinking about children negatively, or is it more just about the skills you have, so you judge people who don't have them more harshly?" she asks.

Despite the long and rich history of "kids these days", remarkably little research has been done on why such opinions exist. These new findings are thus an intriguing start to a problem that will likely never fade.

The study was published in Science Advances.