Causes of Left and Right Handedness

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Literature Review
The Causes of Left and Right Handedness
There are many theories posing the cause of handedness. Many researchers have debated these theories but are yet to have found an unambiguous answer. Even so, it has been found the vast majority of people, around 90%, use solely their right hand for tasks involving dexterity, indicating that only around 10% of the population are left handed and ambidextrous to some degree. Generally is more common in males than in females. Correspondingly, other primates also show strong tendency to be right handed. Vast ranges of testing techniques have been used to assess handedness. There are preference and performance tests, writing hand and self-report which are two of the most popular techniques. Several theories have been advanced over the years to explain the causes of handedness and, in particular, left-handedness. Reasons put forth by various researchers to try and explain these findings are genetics, such as inheritable traits, environmental factors, birth risk events such as stressful births and biological contributions such as variations in cerebral dominance.

The theory of genetics has been examined by many theorists who believe that genetics is the key contributing factor to the cause of handedness. The only genetic models that successfully explain the family incidence data are those of Annett and McManus, which share the feature of including a random component reflecting the biological occurrence of unpredictable irregularity. The models have each been modified to explain the greater incidence of left-handedness in males. The most predominant theory of handedness is Annett's (1972) right shift theory. The theory poses that a single autosomal gene labeled the right shift (rs+) gene if dominant in a person carrying at least one copy were mostly right-handed and those absent from carrying the gene would have a and fifty-fifty chance of being either right-handed or left-handed. The model allows for a...
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