Left-handers were rarer years ago, according to some estimates, consisting of only 2 percent of the population in about 1860 and 4 percent in 1920. Some attribute the growth to today’s 10 percent to a decline in the once-common efforts to force youngsters to use their right hand when learning to write or use cutlery and to an increase in the variety of available left-handed gadgets and tools — scissors, can openers and oven mitts, for instance — as well as musical instruments and sports equipment. But why are some people lefties? Research has suggested a variety of factors, including specific genes, birth weight, mother’s age, preterm births, multiple births, ultrasounds during pregnancy and more. Still, “the biological basis of hand preference . . . remains largely unexplained,” wrote the authors of research published this year in Scientific Reports. Preferring one hand over the other could be simply random variation. Studies have shown, however, that males are more likely than females to be left-handed. This Tuesday, International Lefthanders Day celebrates all who choose sinistrality, the medical term for left-handedness.