A couple months ago in Moscow, a billboard featuring giant breasts caused more than 500 car accidents. Nobody was seriously injured, but it did reinforce a stereotype about guys' tastes.

The truth is more complicated, however. Different body types, including smaller-than-average breasts, attract different people. (Just one more reason why women should worry less about their bodies.) Here are the scientific explanations why...

Size is a relative concept

For a 2013 study published in the Archives of Sexual Behavior, researchers showed male participants five computer-generated 3D images of the same woman, each with a different breast size. Given the choice, "medium" was the most common that men desired, while a quarter chose "large" and a quarter chose the smaller end of the spectrum.

The researchers stressed to us that the breast sizes in the study can't be compared to actual bra sizes for several reasons, including that 3D computer-generated images may not translate to real-life people. The sizes from big to small "are relative descriptive terms."

The important takeaway is there's no objectively ideal size; all preferences are totally normal.

Evolution favors variety

You've heard the saying "men are pigs" or "men are dogs." Nope! But humans are definitely still animals, which means we evolved largely based on mating preferences -- and the more preferences, the better our chances of survival.

To Jason Young, an evolutionary psychology professor at Hunter College in New York City, every breast size preference makes equal sense. "As a general evolutionary rule, diversity is important to reduce too much competition for any one goal," Young tells MTV News. "Having different men attracted to different breast sizes 'evens out the playing field' so that there are more winners; more males meet women with the breast size they desire."

Some guys want to mate with women who have bigger boobs, some smaller boobs, and that's possibly why our species didn't go extinct back in the days of saber-toothed tigers.

Income or appetite may affect breast size preference

Guys, your ideal size isn't necessarily set in stone. In fact, it may change based on your employment status or even whether you forgot breakfast.

According to British researchers Dr. Viren Swami and Dr. Martin Tovee, men's levels of "resource security" -- how comfortable they feel about their ability to survive -- predict what kind of boobs they like. The researchers' experiments found that wealthy guys, as well as guys who had just eaten, rated smaller breasts as more attractive.

The less dough a dude had, or the hungrier he was, the more he found larger boobs attractive, "as large size would be an honest signal of access to resources."

It may reflect how guys feel about women in general

Men who strongly prefer a woman with large breasts might not always like the rest of her. Swami and Tovee conducted another study that found "men’s preferences for larger female breasts were significantly associated with a greater tendency to be benevolently sexist, to objectify women, and to be hostile towards women."

Of course, that doesn't apply to all men with that specific preference; it's correlation, not causation. In some cases, though, being a boob guy could be the opposite of being a nice guy.

Tastes change over time

Why else might a guy prefer smaller-than-average boobs? Dr. Swami tells us it could be due to "an acknowledgement that beauty comes in all shapes and sizes, ideal preferences being shaped by current or previous partner breast size," among other possible causes.

You're not quite the same person as the years go on. Golf might not be the only thing you'll be surprised to enjoy in the future.

Most guys are happy to look at boobs of any size

Researchers in New Zealand who conducted a 2009 eye-tracking study expected to discover that men would spend more time looking at larger-than-average boobs than smaller-than-average. However, they were shocked to discover that "variations in breast size had no significant effect on eye-tracking measures," including "number of fixations and dwell times."

Additionally, the researchers note, the majority of men consider other qualities as more important to "female attractiveness" than breast size itself. As Michael Castleman writes in Psychology Today, "[M]any men insisted that when they fell in love, they fell in love with the women’s breasts, whatever their size."

In other words, guys really are looking at what's on the inside.