We often say in psychology that correlation does not imply causation. In other words, just because there is a mathematical relationship between two variables this does not necessarily mean that one directly causes the other. When we measure the relationship between two variables we arrive at a correlation coefficient, a figure between 0 and 1 or −1, where 0 means there is no relationship and 1 means there is a very strong relationship.
There is, for example, a modest positive correlation between head size and IQ: people with larger heads tend to have slightly higher IQ than those with smaller heads. One obvious interpretation of this is that having a larger head causes people to develop a higher IQ. However, that is just one interpretation of the statistic. It is also conceivable (though it isn’t obvious how) that having a high IQ might lead to having a larger head. More likely, a third factor such as good nutrition in childhood influences both head size and IQ.
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