Women prefer similarity in subtler ways as well: A woman shows a small but highly statistically significant preference for a man who uses similar adjectives to describe himself, 
with “physically fit,” “intelligent,” “creative” and “funny” having the strongest effects. Men showed no such preference.
There are some nuances here. For another thing, the matches people message depend on the options eHarmony’s algorithm gives them, and that sample is skewed toward similar people. Jonny Beber, an eHarmony scientist, explained to me that the algorithm tries to optimize immediate attraction and long-term compatibility, and that because the company believes that “opposites attract … and then attack,” this usually means pairing similar people.
The eHarmony data I used is incomplete: It includes no gay couples, because eHarmony does not make same-sex matches on its main site. He also noted that there were differences in what traits matter to gay people, something the online dating site OkCupid has also found: Gay men and women differ from straight people in their racial preferences, for example.
eHarmony’s data set does show us that in addition to preferring similarity across traits, women seem to know that their preferences are stronger. Before feeding their choices into its algorithm, eHarmony asks users to rate how strongly they feel about nine traits – among them age, ethnicity and religion – and women express stronger preferences for every one.
But Beber has studied data from the company’s same-sex dating site, Compatible Partners, and said similarity predicts long-term relationship satisfaction in gay couples, just as it does in straight couples
This got me wondering, how self-aware are people in general? Does what they claim they care about align with their messaging behavior? It often does. People with high incomes and high degrees of education claim that income and education matter to them more, and they display an especially large messaging preference for potential mates with high incomes and educations. Members who say religion matters more to them show stronger preferences about their match’s religion.
Messaging may not be an honest reflection of attraction if the people doing the messaging fear rejection (although economists have found that such “strategic behavior” is minimal in online dating)
But for other traits people appear to be confused, or lying. People of every age claim that age matters to them about the same amount – they rate it about 4.5 on a scale where 1 is “not at all important” and 7 is “very important” – but older men show much stronger age preferences in whom they message. Everyone claims that ethnicity matters to them about the same amount (4.2), but some ethnicities show much stronger preferences. Men are more likely to message women who drink more even if they claim to want women who don’t drink at all. (This remained true even when I controlled for attractiveness, age and whether the woman messaged the man, and even when I looked only at men who rated their drinking preference as important.)
So we can break down the general idea of “birds of a feather flock together” even further, into two patterns:
- The simple pattern: People who display a certain trait prefer other people who display that trait; people who don’t prefer people who don’t.
- The subtler pattern: Everyone prefers people with a certain trait, but people who have the trait themselves display a stronger preference for other people with that trait.
Height illustrates both these patterns. Men follow the first: Short men prefer short women, and tall men prefer tall women. Women follow the second: All women prefer taller men, but tall women display a stronger preference for tall men. For intelligence, women follow the first pattern: Those who describe themselves as intelligent prefer men who describe themselves as intelligent, and women who don’t prefer men who don’t. Men follow the second pattern: All men prefer women who describe themselves as intelligent, but men who describe themselves as intelligent display a stronger preference.