Image copyright Getty Images Image caption During the teenage years, there are important changes going on inside the brain

Brain immaturity during adolescence could explain why some teenagers fail to respond to incentives such as cash rewards.

Adults are good at putting maximum mental effort into the things that matter most.

But, brain circuits are still developing in teenagers, making it harder for them to tackle meeting their goals, say US psychologists.

Attempts to improve student grades with money have had mixed success.

The research, published in the journal, Nature Communications, shows that brain connectivity continues to develop throughout adolescence, affecting teenagers' ability to perform when the stakes are high.

In the study, researchers at Harvard University used functional magnetic resonance imaging (fMRI), which measures brain activity by detecting changes linked to blood flow.

Young people, aged 13 to 20, were brain scanned while they played a computer game.

They played for high stakes, when they could earn $1 for correct responses or lose 50 cents for incorrect responses, and for low stakes, when they could earn 20 cents or lose 10 cents.

Lead researcher Katie Insel said older adolescents were able to boost their performance when the stakes were high.

However, younger adolescents performed similarly for low and high stakes outcomes.

''These findings demonstrate that brain connectivity continues to develop across adolescence,'' she told BBC News.

''This means that as teens age, they become better at adjusting brain connectivity across motivational contexts, which in turn allows them to do better when working towards a high-value goal.''

Past studies have shown that connections between different parts of the growing brain take years to develop.

The last bit of the brain to reach full maturity is the prefrontal cortex, which is responsible for things like planning, controlling emotions and empathy.

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