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Forget expensive educational DVDs and private tutors, the secret to smart children could be as simple as giving birth to them two years apart.

Researchers who studied thousands of children found a two-year gap to be optimum in boosting brain power.

Any shorter, and the reading and maths skills of the older child dipped.

The effect was strongest between the first and second-born, but siblings in bigger families also benefited.

The theory comes from Kasey Buckles, an economist whose own children are, rather fortunately, just over two years apart in age.

She said it is likely that the difference in academic achievement is linked to the time and resources parents can invest in a child before a younger sibling arrives.

However, waiting more than two years did not increase the advantage, the Journal of Human Resources will report.

Siblings with a two-year spacing include Albert Einstein and sister Maja, and Lord Attenborough and younger brother David.

Kasey Buckles, who lead the study told the Sunday Times: 'We believe this is the first time anyone has established a casual benefit to increase the spacing between siblings.'

The study also showed that gaps between children in larger families was also beneficial.

Buckles told the newspaper: 'The two year gap is significant because the early years are the most important in a child's development so dividing your time when the child is one is more harmful than dividing it when the child is already at school.'

The effect was more pronounced in families with lower incomes, as those with more money could spend to compromise for lack of time.

(Read by Emily Cheng. Emily Cheng is a journalist at the China Daily Website.)

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(Agencies)