|A 2015 screenshot of eChemistry.com, an online dating site that purports to match people scientifically.|
The dominant site in this category is Chemistry.com (Alexa-rank: 7,171), motto: “lets people experience real chemistry”, a subsidiary of Match.com (Alexa-rank: 310), launched in 2006 and developed, in large part, on the theories of American anthropologist Helen Fisher, which claims to match people according to compatibility and chemistry.  In 2008, Chemistry.com was the fourth largest online dating site, based on number of singles available; largely due to a successful ad campaign targeting eHarmony (#5 largest dating site) and their anti-gay Christian-only, who regect people who aren't happy all the time (adjacent video).  Chemistry.com uses information such as middle-finger-to-ring-finger length ratios (digit ratio), an indication of testosterone levels, and personality type matching assessments, such as by asking people "do you like to count things"; counters have high dopamine levels and tend to be the "explorer type".
The first science-based matching site to pioneer the method of having matchees send in samples of their body composition, is ScientificMatch.com (Alexa-rank: 1,011,265), motto: “the science of love”, launched, in December of 2007, by American mechanical Engineer Eric Holzle.
By having singles send in saliva samples, the site facilitates a laboratory analysis of each person's immune system type and, using this data, claims to create optimized “physical chemistry” or "sexual chemistry" between people based on the sweaty T-shirt study, a pattern, discovered in 1995, which finds that people are most attracted to the smell of people who have the most-dissimilar immune system.  The site was conceived by Holzle, a long-time internet site dater, after watching a TV discussion on the findings of the sweaty T-shirt study.
The theory of desired dissimilar immune system matching can be quantified according to markers on a person’s major histocompatibility complex (MHC), a large gene region that controls the immune system response, and postulates that couples attracted to this type of scent owing to the result that a resultant child would create a more robust immune system, more defensive against a greater variety of pathogens.
In the mid 1970s, MHC-dissimilar tendency matching was shown to be the case for mice (and later for other animals such as fish) and in 1995 Swiss biologist Claus Wedekind, creator of the sweat T-shirt study, proved that the pattern holds for humans.  In this study, Wedekind had a group of female college students smell T-shirts that had been worn by male students for three nights, without deodorant, cologne or scented soaps. Overwhelmingly, the women preferred the odors of men with the most dissimilar MHCs to their own (see: adjacent video).
A third scientific-based matching site is eChemistry.com (Alexa-rank: 577,687), motto: “the science of attraction”, started in 2006 by American chemical engineer Glenn Gasner, which claims that by using the new ‘science of attraction’ they have ‘taken the mystery out of romantic chemistry using nine years of research to match single users using personality patterns scientifically isolated in old high-chemistry couples.’  Gasner’s site algorithm, however, is a synthesis of probabilities, e.g. how may dates does one need to go on to find a successful match, and personality inventories, e.g. the Five Factor Model. The only science Gasner claims to use is the scientific method; subsequently, the logic of a standard chemistry textbook is not part of his scheme. 
The site eHarmony.com (Alexa-rank: 1,100) founded in 2000 by American psychologist Neil Warren and Greg Forgatch, whose vision is not simply to create marriages, but to create happy marriages by using scientific research to unite compatible individuals. In a 2007 interview of the site’s current CEO Greg Waldorf, self-taught computer programmer and Harvard business school graduate, he states "I love the challenge, that people think: How could you possibly measure interpersonal chemistry? I like the idea of research and measurement around these things that seem so impossible to quantify." 
Wall Street Journal editor Emily Parker, who interviewed Waldorf, reasoned to herself “of course, this only leads one to wonder, how does one measure interpersonal chemistry?” She found that, “while eHarmony is launching an ambitious research project to do just that, Mr. Waldorf admits that they still haven't solved the puzzle.” Moreover, according to Waldorf, "we can help them figure out their compatibility, but individuals have to figure out their chemistry. The nice thing is that on eHarmony, you're starting out with a pool of matches with whom you at least have compatibility."
The popular social dating site PlentyofFish.com (Alexa-rank: 638) has, what they call, a "relationship chemistry predictor" (algorithm), which measures (a) self-confidence, (b) family orientation, (c) self control, (e) social dependency/openness, and (f) easygoingness, to determine supposed “chemistry” in relationships. 
Another connected site is BioLoveMatch.com (Alexa-rank: 904,261), started in 2000, which claims to use biorhythm compatibility to match potential loves. This site, however, seems to only ask for birthdates of pairs.  In the seduction community, a site called TheApproach.com, motto: “the science of social chemistry for the modern gentleman”, started in 2005 by Vincent DiCarlo and Sebastian Drake, teaches men how to approach women. 
The original application of “science” to the matching of singles, via the Internet, was the use of personality assessments, e.g. the Big Five, the one most often employed by psychologists to access personality, to better match individuals. Compatibility testing and long-term relationships, in particular, started in the mid-1950s by the “Father of American Marriage Counseling” Paul Popenoe and was later refined and popularized in the mainstream by Glenn Wilson and Jon Cousins. 
The use of the harder sciences, such as evolutionary psychology, neuroscience, genetics, chemistry, neurochemistry, biochemistry, or physical chemistry, etc., however, to facilitate the successful matching of singles, is a relatively new field of research. This fact, together with the typical “black box” algorithm procedures used by such sites and the lack of published scientific studies about results, compounds the credibility factor determinations of scientific-matching.
A few criticisms, however, have merged. In the 2007 article in the OnlineDatingMagazine, former chief psychology officer for True.com, James Houran supposedly “debunks scientific matching”.  In short, according to Houran, who seems to be targeting Chemistry.com, “the dating industry would be better off validating personality tests and matching algorithms than worrying about background checks.” In sure reference to Pepper Schwartz, PerfectMatch.com, sociology professor (University of Washington), author: Finding Your Perfect Match - 8 Keys to Finding Lasting Love through True Compatibility (2006), and Helen Fisher, Chemistry.com, anthropology professor (Rutgers University), author: Why We Love - the Nature and Chemistry of Romantic Love (2004), he concludes:
“What we have are groups of authors, nice sounding university affiliations and academicians generally interested in relationships. Yet, nowhere do we see these teams with published, compatibility experts, and more importantly, tests and measurements experts who ground their work in the statistical gold standard of modern test theory!”
● Human chemistry
● Love the chemical reaction
● Endorphin theory of love
1. (a) Thims, Libb. (2007). Human Chemistry (Volume Two), (ch. 11, section: “Online Matching”, pgs. 455-64). (preview), (Google books). Morrisville, NC: LuLu.
(b) Gottlieb, Lori. (2006). “How Do I Love Thee?” (A growing number of Internet dating sites are relying on academic researchers to develop a new science of attraction. A firsthand report from the front lines of an unprecedented social experiment.) The Atlantic Monthly, March.
(c) Popular Match System Dating Sites – DatingSitesReviews.com
2. Chemistry.com - homepage
3. Comparison of the Top 10 Dating Sites (2008) – MyOnlineDatingService.com
4. (a) ScientificMatch.com – homepage
(b) Physical chemistry defined – ScientificMatch.com
5. Wedekind, C. et al. (1995). "MHC-dependent preferences in humans." Proceedings of the Royal Society of London 260: 245-49.
6. (a) eChemistry.com – home page
(b) Romantic chemistry (defined) - eChemistry.com
(c) Origin of eChemistry.com – Perex.com
7. (a) Gasner states that he calculated this ratio via the ‘obvious math: (1/10)x(1/10)x(1/10) leading to the conclusion that 1 in 1,000 females was a marriage-level match’. Assuming a reciprocal nature in both physical and personality, however, he seems to be missing a 1/10th factor in this calculation, which if used correctly would yield 1 in 10,000 females at a good match.
(b) The supposition that Gasner uses no “chemistry textbook” theory is difficult to determine exactly, being that Gasner stopped returning emails on 01/31/06 with American chemical engineer Libb Thims, after being questioned about the nature of his website and specifically if he used any sort of thermochemistry or chemical thermodynamic theory in his matching algorithms.
8. BioLoveMatch.com – homepage
9. TheApproach.com – homepage
10. Houran, James. (2007). “Marriages from Online Dating Sites … Enough Already!”, OnlineDatingMagazine, June
11. Staff writer. (2007). “Dr. James Houran Debunks Scientific Matching”, Online Dating Insider, July 02.
12. Parker, Emily. (2007). “The Matchmaker: Log on, Subscribe, Meet, Marry”, The Wall Street Journal Online, The Weekend Interview, 10 Feb.
13. Personality Relationship Chemistry Test – PlentyofFish.com
● Frazzetto, Giovanni. (2010). “The Science of Online Dating”, EMBO, 11: 25-27.
● Science Connection (dating site) – SciConnection.com (1991-2010).