In the country of Boldavia at present, only 1 percent of 80-year-olds are left-handed, whereas 10 percent of 50-year-olds and 15 percent of 20-year-olds are lefthanded. Yet over the past 80 years, the proportion of Boldavians who are born left-handed has not increased, nor have Boldavian attitudes toward lefthandedness undergone any important changes. Which one of the following, if true, most helps to explain the variation in incidence of left-handedness among Boldavians? ['In Boldavia, left-handed people are no more likely than right-handed people to be involved in accidents', 'Ambidexterity is highly valued in Boldavia.', 'Left-handed people have never accounted for more than 18 percent of the population of Boldavia.', 'In Boldavia, where men have a shorter average life expectancy than do women, lefthandedness is less common among women than among men.']