Here's a seemingly silly question: Is 571 a he or a she? Deep down, you probably think of it as masculine. When people look at gender-ambiguous prompts like babies' faces, they're more likely to think of them as male when they're paired with an odd number, reports a study led by James Wilkie of Northwestern University. The finding suggests that our beliefs about gender may creep into everything from phone numbers to pricing—but why?

1. We instinctively gender everything. Gender shapes many of our experiences from birth, and we're constantly noting the difference between objects intended for men or women. Such deeply ingrained distinctions mean we instantly assign a gender to everything—even numbers.
2. A pair seems more nurturing than a singleton. We associate the number 1 with agency and autonomy—stereotypically male traits. The associations we have with 2—relationships, community, caregiving—are more feminine and seem to extend to all even numbers.
3. We use concrete metaphors to understand abstract concepts. Physical up-and-down language dominates business talk (higher-ups, the corporate ladder) even if everyone works on the same floor. Similarly, we may use the concrete male/female split to help us make sense of the theoretical odd/even dichotomy.
4. Even numbers are less cognitively taxing. We can easily count to 100 in even numbers, but counting to 99 in odd numbers is trickier. Mental fluency, Wilkie suggests, might be related to gendered numbers: Things that are easier to think about feel friendlier and more comfortable—traditionally feminine traits.