Because Iwanted all right handed children I always held them by the right hand when walking and always made them grasp an object with their right hand so that they would eventually automatically use that hand. It may not be a scientific answer to your question but it worked for my 5 kids.

Most humans are right-handed, a minority are left-handed. This appears to be universally true for all human populations anywhere in the world.In 2007, researchers discovered LRRTM1, the first gene linked to increased odds of being left-handed. The researchers also claim that possessing this gene slightly raises the risk of psychotic mental illnesses such as schizophrenia. This is an evidence for genetic influence for handedness; however, it is non-Mendelian, and geneticists can not agree on the exact process. There is also evidence that handedness can be influenced and changed by social and cultural mechanisms. For instance, teachers force children to switch from using their left hand to using their right hand for writing.

The following answer has been selected and edited by New Scientist staffThe simple answer is that they've inherited genes for left or right-handedness, which is why handedness runs in families and identical twins are more likely to have the same handedness than dizygotic (fraternal) twins. The genes involved are a little strange, because while one makes people right-handed, the other only makes it random as to whether an individual is right or left-handed. So identical twins with the latter gene can have different handedness.Genes are only the immediate cause of handedness. Very occasionally, "biological noise" during development, or brain or arm trauma, will override genes and cause "pathological handedness".Why humans alone among animals are 90 per cent right-handed is a separate question, with the answer going back 2 million years. This is when human brains became asymmetric and the neural equipment for the fast, precise movements for speech and finger dexterity became localised in the left hemisphere. Why it is the left hemisphere is unclear.Yet another question is why some people are left-handed. The answer is that there must be advantages to having the genes for left-handedness, although the advantages are still to be discovered. Finally, why aren't all animals ambidextrous? Most likely because it pays to specialise - if all practice is with just one hand, that hand will be more accomplished than a hand that only benefited from half the practice time.Chris McManus, Professor of Psychology and Medical Education University College LondonChris McManus is the author of Right Hand, Left Hand (Weidenfeld and Nicolson, 2002)- Ed

Chris McManus says "There must be advantages to left-handedness, though they have yet to be discovered" Not so, they have. The builders of Scotland's brochs spiralled their stairways so the defender at the top had the advantage of the attacker, whose sword arm was hampered by the wall. If one was attacking, left-handedness was a decided advantage.

To combine two of the concepts mentioned -Family history has it that I preferred left hand as a baby and was trained out of it by similar methods to those mentioned; I have a weaker right preference than most people I have observed. I used this to advantage when I learned to fence - I intentionally learned left-handed for the advantage it gave me.Even in other things, I am sometimes thought to be left-handed; I use a mouse with my left hand by habit because it makes more sense; it leaves my right hand free for the keyboard without having to let go of the mouse, and places the components in a more sensible arrangement.