A lot of charities and welfare groups are built around helping the needy, but without controlled conditions and long-term monitoring, it's been pretty much impossible to scientifically measure the effect that some regular financial help has on families.

But a study in the US has accidentally done just that, and in the process it's revealed how life-changing it is for the poorest children in society when their parents are given a little extra income each year, without having to work longer hours.

The Great Smoky Mountains Study of Youth began 20 years ago, with the intention of simply observing the mental health of 1,420 low-income children living in North Carolina in the US. But four years into the study, roughly a quarter of the participants' parents received an unexpected increase in income of around US$4,000 a year per person.

These parents were members of the Eastern Band of Cherokee Indians, and the extra income came from a new casino built on the reserve, providing them with a share of the profits. And while $4,000 a year may not sound like that much, for these families it was enough to boost their income by an average of almost 20 percent, which shows just how hard they had it beforehand.

But what was even more impressive, from a scientific point of view, is that it provided scientists with an insight into what happens to children when you change nothing but their parents' income. And the results were pretty incredible.

"This was hugely important to the development of the children, to their wellbeing” lead researcher Randall Akee from the University of California, Los Angeles, told Roberto A. Ferdman over at The Washington Post. "And the effect wasn’t small either - it was actually fairly large."

Their analysis showed that children whose parents had received the income boost were less likely to have behavioural and emotional disorders compared to their peers.

But it wasn't just mood and mental health that was improved - the researchers also found that the extra cash boosted two personality traits in children that are linked to long-term positive life outcomes, as Ferdman reports:

"The first is conscientiousness. People who lack it tend to lie, break rules and have trouble paying attention. The second is agreeableness, which leads to a comfort around people and aptness for teamwork. And both are strongly correlated with various forms of later life success and happiness."

Interestingly, the children who benefitted the most from the extra money were those who were the most behind for their age group when the study first began.

"This actually reduces inequality with respect to personality traits," said Akee. "On average, everyone is benefiting, but in particular it's helping the people who need it the most."

As always, with these types of observational studies, the results only show correlation, not causation, so the researchers can't say for sure why the children benefitted from the extra money.

But the scientists did find some evidence that might explain the change. For starters, relationships between parents improved after the income boost, usually because there was less stress about money.

And some families used the extra cash to move to areas where people make more money and where children are more educated in general, so that could have positively influenced the emotional children's development.

More research will need to be done to tease out these links, but what's important is that this once-in-a-lifetime study has shown for the first time that providing the poorest in society with a little extra income really can have significant benefits for the next generation.