As with many developmental milestones, there is no single age 'engraved in stone' by which time a child would be definitely righty or lefty. Generally speaking though, you would be able to figure out your baby's handedness at around 2 years age.
While your baby's preference for her right or left hand may start to appear as early as 6 to 9 months of age, this is unlikely to be consistent. Some children may be ambidextrous (using both hands equally) until they're 5 or 6, when they finally make a choice.
Do remember, hand dominance is greatly influenced by genetics. If both you and your partner are left-handed, your child has a 45 to 50 percent chance of being left-handed as well. (In the general population, around 10 percent of people are left-handed.) Statistically, the identical twin of a left-handed person has a 76% chance of being left-handed, identifying the cause(s) as partly genetic and partly environmental.
TIP: If your baby seems to be using one hand exclusively before he's 18 months old, however, talk to your pediatrician, as early hand dominance may be a sign of neurological problems.
The nature versus nurture debate ... While genetics alone don't entirely explain why someone ends up right or left-handed, hardwiring of your child's nervous system is at least part of the reason. Forcing him to use his right hand when he's really a lefty is unlikely to work in the long run and will only confuse or frustrate him.
Remember, while being lefthanded has been associated with social stigma, especially in India, studies have suggested that the proportion of left-handers is increasing and left-handed people as a group have historically produced an above-average quota of high achievers, and among college-educated people, left-handers earned 10 to 15 % more than their right-handed counterparts!
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