If there’s one thing scientists have to be good at, it's observation. Yet centuries of research have overlooked one very obvious fact: Male and female animals are biologically different. A new study published in the journal Nature Communications demonstrates just how much that oversight has shaped and limited our knowledge of the world.

It’s standard practice in biology and medical research to experiment on just one sex, usually male. That could be male cells, or male mice, or male humans. Whatever the organism, it's assumed that the results are universal and equally applicable to the other half of the species—an assumption that has led us to overlook new species, prescribe dangerous doses of medicine, and design deadly seatbelts.

In 2014, the National Institutes of Health announced that it would require scientists to include both sexes in future studies. This announcement was met with resistance. Researchers don’t want to change the way they design experiments. Including both sexes is expensive, they say, and female organisms are unpredictable. (We're not even joking. We wish we were.)

But ignoring huge variables like sex is not the best way to do science. It’s certainly not the best way to get valid, useful results.

To demonstrate this, one team of scientists decided to prove just how different the sexes’ biology can be. They collected data on more than 50,000 lab mice, marking each one on 234 different physical characteristics like bone mass, behavior, head shape, and coat health. Most of the mice (40,192) had been bred with some gene mutation that could be switched on or off.

Comparison of all these traits made a pretty strong case for sex-balanced research. For non-mutated mice, sex affected 56.6 percent of quantitative traits like bone mass and 9.9 percent of qualitative traits like normal head shape. Sex also changed how the mutant groups’ mutations were expressed, and how those mutations changed the mice’s bodies.

Co-author Judith Mank from University College London says it's time to give up on imbalanced research. "This study illustrates how often sex differences occur in traits that we would otherwise assume to be the same in males and females," she said in a statement.

"More importantly, the fact that a mouse's sex influenced the effects of genetic modification indicates that males and females differ right down to the underlying genetics behind many traits. This means that only studying males paints half the picture."