We’ve all heard it before: “Stay positive, look on the bright side — don’t let things get you down.” but the fact of the matter is it’s way easier to say it then to actually train yourself to be more positive. Interestingly enough, the reason for this could be tied into our biology as humans.

According to a study featured in the The Review of General Psychology and the New York Times, negative thinking is likely a huge factor in why humans survived as a species, since our ancestors who were more attuned to the dangers and bad things around them were the ones more likely to survive and pass on their genes. Basically, we may very well be programmed to have trouble seeing the positives in life and looking on the bright side.

The problem with negative thinking is that studies also show that this type of mindset and outlook on life can adversely affect our health. According to a study published in the Public Library of Science, negative thinking and self-blame can be the main predictor of many mental health problems, and research featured in Mental Health America states that it can even have a negative impact on our lifespan.

Basically, staying positive doesn’t just make us happier, but it could actively be helping to keep us healthy.