Sometimes opposing adages fight to a draw: “Better safe than sorry” versus “Nothing ventured, nothing gained.”

But when it comes to “Opposites attract” versus “Birds of a feather flock together,” the data are in: We end up with partners like ourselves.

A study of 291 newlywed couples found spouses to be closer in values, religiosity, and political attitudes than would be predicted by chance [1].

Scientists have a term for this: positive assortative mating. (It’s negative assortative mating when opposites attract.)

The human species isn’t the only one that flocks together. A meta-analysis of assortative mating in animals based on traits such as size and color found that nearly all the assortment was positive [2]. Not that sorting by size and color is limited to animals: humans tend to marry people with a similar level of body fat [3], and online daters stick to their own race [4].

People also gravitate toward mates whose faces look like theirs. In one study, subjects who were presented with a series of photos were able to pair a woman’s image with that of her partner, based on facial similarities — even when only isolated features (noses, mouths, eyes) were displayed [5].

Friendly people apparently seek same: A field experiment in rural Senegal found spouses to have corresponding levels of generosity [6].

People with less desirable qualities also attract one another. Having bipolar disorder or major depression makes you more likely to marry someone else with an affective disorder [7]. Alcoholics, too, tend to pair up, with potentially disastrous results for their future offspring [8].

Perhaps most consequential, we sort ourselves by socioeconomic status. On this count, income similarity matters, but similar schooling seems to matter more, maybe because it strongly implies cultural commonalities [9]. The results of class-based mating are profound.

A 2014 study examined the growing tendency of Americans to partner with people of similar educational backgrounds, and found a clear connection between this shift and the growth in household income inequality between 1960 and 2005 [10].

Some of this sorting can be explained by shared environments. For example, religious people meet in church and wealthy people meet in college. Market forces also play a role: Couples match in attractiveness because the tens pair up, leaving the nines to settle for each other, and so on.

Still, we nonmodels make do. One study found that less attractive people realize certain dates are out of reach and adjust priorities accordingly, learning to value traits like a sense of humor [11].

A final factor in sorting is intrasexual competition. Take the mangrove snail: Males prefer larger females, and when two males mount a female’s shell, they fight it out, with the larger male winning the prize [12]. Tens with tens. Unfortunately, the loser can’t comfort himself by finding a mate who’s really funny.

The Studies:

[1] Luo and Klohnen, “Assortative Mating and Marital Quality in Newlyweds” (Journal of Personality and Social Psychology, Feb. 2005)

[2] Jiang et al., “Assortative Mating in Animals” (The American Naturalist, June 2013)

[3] Speakman et al., “Assortative Mating for Obesity” (The American Journal of Clinical Nutrition, Aug. 2007)

[4] Lewis, “The Limits of Racial Prejudice” (Proceedings of the National Academy of Sciences, Nov. 2013)

[5] Alvarez and Jaffe, “Narcissism Guides Mate Selection” (Evolutionary Psychology, 2004)

[6] Tognetti et al., “Assortative Mating Based on Cooperativeness and Generosity” (Journal of Evolutionary Biology, May 2014)

[7] Mathews and Reus, “Assortative Mating in the Affective Disorders” (Comprehensive Psychiatry, July 2001)

[8] Grant et al., “Spousal Concordance for Alcohol Dependence” (Alcoholism: Clinical and Experimental Research, May 2007)

[9] Kalmijn, “Assortative Mating by Cultural and Economic Occupational Status” (American Journal of Sociology, Sept. 1994)

[10] Greenwood et al., “Marry Your Like” (American Economic Review, May 2014)

[11] Lee et al., “If I’m Not Hot, Are You Hot or Not?” (Psychological Science, July 2008)

[12] Ng and Williams, “Size-Dependent Male Mate Preference and Its Association with Size-Assortative Mating in a Mangrove Snail, Littoraria Ardouiniana” (Ethology, Oct. 2014)