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Researchers often compare the differences between identical and fraternal twins to better understand health and behaviour.
The first major insight is that genes and environments almost always combine to influence our life trajectory. Sometimes the largest factor is genetics (think genetic disorders). Sometimes it is the environment (think infections). Mostly, it is somewhere in between.
Such studies have accelerated the search for genes and environmental agents that cause or trigger diseases. This has helped us understand, treat and even prevent diseases. As twin research has matured, it has progressed to addressing important questions about when and how diseases originate.
So what has research from twins taught us about specific diseases and the human body?
Most studies linking environment and disease are complicated by genetic factors. To get around this, we can work with twins who differ in environmental factors.
One such Australian study from 1994 compared 20 pairs of female twins in which only one of each pair was a long-term, heavy smoker.
The researchers found smoking one pack of cigarettes a day for 20 years resulted in sufficient loss in bone density to cause osteoporosis. This doubled the risk of having a bone fracture.
This provided compelling evidence that smoking causes osteoporosis and an increased risk of bone fractures.
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