Letters to the Editor Readers discuss the biology of sex, plastic in the brain and more By Science News Staff 2 hours ago It’s a matter of size A January executive order by President Donald Trump designates people as female if they make the “large” reproductive cell (the egg) and male if they make the “small” one (the sperm). But the human sexes don’t fit neatly into a male–female binary due to factors such as genetics and hormones, senior molecular biology writer Tina Hesman Saey reported in “The real biology of sex.” Reader Root Gorelick, a biologist at Carleton University in Ottawa, appreciated the feature and wrote in to add some nuanced points. Sign up for our newsletter We summarize the week's scientific breakthroughs every Thursday.