In the past 20 years, online dating has become the preferred way to find a mate in much of the Western world. In 2000, a few hundred thousand people used the internet to hunt for romantic attachments. Today that figure is well over 300 million.

This change has significant implications for anthropologists who study human mating patterns. In the past, this research has suffered badly from the lack of good data in sizable quantities. But all that has changed with the data from dating websites.

All of a sudden, anthropologists can see who is messaging whom and who replies. That is beginning to provide unprecedented insight into the nature of human mate selection. So what does it show?

This map of the US shows how online daters tend to only message people who live in roughly the same region—which makes sense.

Today we get an answer thanks to Elizabeth Bruch and Mark Newman at the University of Michigan, who have studied the behavior of some 4 million active users from a popular (but unnamed) online dating site. Their analysis reveals much that is expected—men tend to initiate contact, for example. But it also shows curious, unexplained differences in how people date across America.

The researchers collected messages sent between heterosexual individuals during January 2014. They then studied the network that these messages created. In this network, users are nodes, and a link exists between two of them if a message has been sent between them. In particular, the researchers focused on reciprocated messages. “Reciprocal interactions we take to be a signal of a baseline level of mutual interest between potential dating-partners messages,” say Bruch and Newman.

The researchers began their study by looking for communities within this network. These are areas of the network that have a higher density of links than would be expected to occur randomly.

That clearly showed how the dating market is geographically stratified. The communities in the network correspond to regions such as New England, the East Coast, the South, Northern and Southern California, and so on.

In other words, people tend to contact potential mates who are nearby. “Few people living in New York will exchange messages with people across the country in California if the primary goal is to arrange a face-to-face meeting with a possible romantic partner,” say Bruch and Newman.

The team then studied the networks in several US cities: New York, Boston, Chicago, and Seattle. In each case, they divided users into four age groups: those in their early 20s, late 20s, 30s, and 40 or over. They also looked at patterns in and between self-identified ethnic groups.

Many of the results are unsurprising. For example, most interactions are between people in the same age groups and ethnic groups; men tend to contact women who are younger than they are, and reciprocated messages tend to be between people of the same ethnicity.

One interesting trend is that the younger age groups tend to be male-heavy, but the mix becomes progressively more female in the older age groups. “The youngest submarkets in Chicago and Seattle, for example, have almost two men for every woman,” say Bruch and Newman.

The researchers suggest several reasons for this. Women tend to marry earlier than men and this depletes their numbers in the younger dating markets. Women also partner with older men and this too depletes their numbers in the younger age groups. The same factors reduce the number of men in the older age groups, which increases the proportion of women.

Beyond this, there are a number of curious results too. For example, in New York the messaging patterns look somewhat different from those in Chicago because New York men pursue younger women on average. Just why isn’t clear.

Another unexplained finding relates to women who initiate contact with men. These women are more likely to receive a response from older men than from younger ones. But when women in the oldest age group initiate contact with younger men, they are more likely to receive a reply from those in the youngest group than they are from those in the second-youngest group. Again, why this happens isn’t clear.

This work provides a unique insight into the complex structure of the online dating market in the US. “Our study illustrates how network techniques applied to online interactions can reveal the aggregate effects of individual behavior on social structure,” say Bruch and Newman.

Ref: arxiv.org/abs/1904.01050 : Structure of Online Dating Markets in US Cities