In the 19th and early 20th century, the percentage of left handed people was about 2.5%. Now it’s 10-15%. A startling growth? Perhaps not. In 1900, we should interpret 2.5% as “1 in 40 people are so left-handed that in spite of strong social opposition, they persist in leftyness.” The people who preferred their left hands but could be converted under pressure were counted as righties, the people who were better with their left hands but could manage with the right hands were counted as righties, the actually ambisinister were counted as righties, and even the people who late in life had disease or injury to their right hands were counted as “really righties but bravely managing to use their left hands anyway”. It’s not surprisingly that the number of “open” lefties has skyrocketed (an increase by a factor of 4 or 5 - when will it end!), and there’s no need to search for a mysterious environment poison causing this - it’s just that we’re naturally measuring two different things: “stubborn and strongly lefthanded people” in 1900, and “people who prefer to use their left hand to any extent” today.