Wonder why some people just happen to be so smart, and it even seems to flow down from generation to generation? There is a reason.

A new study is saying there's a crucial link that ties intelligence to genes, meaning that one is born with the brains.

Intelligence test results and DNA of more than 3,500 people middle-aged and older were compared, and the researchers found that 40% of the differences in intelligence among the people tested was connected with genetic variation.

According to the report, 51% of the variation in fluid intelligence (on-the-spot thinking) and 40% in crystallized intelligence (vocabulary) are associated with genetic differences.

The researchers studied over half a million locations in the chromosomal DNA where people are known to differ, and compared them to the two types of intelligence.

The research was conducted by an international team led by Ian Deary of the University of Edinburgh and Peter Visscher of the Queensland Institute of Medical Research. Their findings were published in the journal Molecular Psychology on Tuesday.

While similar studies of this kind were conducted in the past, this is the first that tested thousands of people and over 500,000 genetic variants.

However, Dreary reminds that early childhood environment and life environment are also important contributors to intelligence differences.

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