have shown that, on average, left-handed people die nine years earlier than right-handed people. There are many theories as to why this is, but the exact cause is still inconclusive due to a lack of detailed data in large populations. A contributing factor may be the slightly higher occurrence of accidents caused by environments and tools not designed for left-handers, but more significant is the evidence that left-handedness is often a marker for birth related stress. A study has shown that, of children born with very low birth weights, more than half developed left-handedness, compared to about 8 - 10% in the overall population. It has also been shown that women who smoke during pregnancy are more likely to produce left-handed offspring. These intrauterine stresses can correlate with a wide variety of health problems many years later, resulting, on average, in an earlier death for a grater number of left-handers.