I am a left handed girl.And the reason is family.We are ''lefties''.

Anyways the reasons why some people are left handed are not clear,but they seem to depend on a mixture of genetic and environmental factors.
left handedness seem to run in families,but it also appears in families with where no immediate member is left-handed.Some theoretical environmental reasons for left handedness are birth trauma,exposure to high levels of testosterone in the womb and physical conditioning.

only 4% of the population is left handed.

One particular theory is something related to two halves of brain.the left half and the right half.each of which functions differently.medical science believes that left half of the brain predominates the right half.the stem of this theory is the fact that nerves from the brain cross over at neck-level to the opposite side of the body.and nerves from the other side of the brain reciprocate.the end result is that the opposite sides of the body are supplied by the opposite of the brain.

The predominant left half of the brain,which graciously supplies the right half of the body,theoretically renders it more skillful in reading,writing,speaking,and working and makes most people right handed.''lefties'' however are the product of an inversion,whereas the right half of the brain predominates,and they work best with the left side of their bodies.

Theory number 2 trickles down to the asymmetrical nature of the body.examples of the asymmetry,which flows from head to the toe,are that the right side of our faces differs slightly from the left,that our legs differ in strength or that our feet vary in size.one aspect of asymmetry is that for most people right hand is stronger than the left.

Lefties are athletically inclined,think more quickly,they have twice the problem solving skills and higher IQ.People who use their left hand to write but favor the right side in every thing else scored the same as ''true''right handed people.

you can check this link for detailed answer.

What causes some people to be left-handed, and why are fewer people left-handed than right-handed?: Scientific American

Our society is just for right handed people.Basic things like scissors,doorknobs,locks,screwdrivers,automobiles, buttons on clothing and musical instruments are just for 96%of the population.

Nobody cares about us.In exams It is we who suffer with those damned right arm chairs.