Attention, guys: pumping up may not get you the girl.

Researchers from the University of Pretoria in South Africa took photos of 69 men of varying body types right after they had given them the hepatitis B vaccine, which acts to trigger their immune system. They showed those photos to a group of women who were ovulating—presumably putting them at their most sexually selective— and a had third group measure the men on their masculinity and body types.

Researcher found men who were rated in decent shape but slim were more attractive to the ovulating women than men who were overweight or severely underweight and the men who had a muscular, traditionally masculine shapes.

The researchers also found that a low (but not too-low) body weight correlates with a strong immune system, which would also make a man more attractive to a fertile woman. Muscles and more masculine features, however, didn't correlate with a strong immune system, which the researchers believe is why those body types weren't found to be as appealing.