Geir Konráð Theodórsson thought he had chemistry with a young woman he was conversing with via the popular dating app Tinder. The conversations were going well, so they decided to move it to Facebook. FB 0.92 % Facebook revealed some mutual friends between the pair: her mother, her grandmother and the sister of her grandmother.
“This is suspicious,” she messaged him.
Mr. Theodórsson, 30, lives in Borgarnes, a town located on a peninsula in western Iceland. It has a population of fewer than 2,000. With their mutual friends signaling a red flag, he logged into Íslendingabók.
Íslendingabók, or the Book of Icelanders, is an online database that contains the full genealogy of 720,000 Icelanders, living and deceased. Assembled by combining old Icelandic genealogy books and church records, it launched online in January 2003 and gives Icelanders an outlet for their craving for genealogy, an ardent hobby for many in the country of 330,000.
Now, as social media and apps expand the dating pool, many people are turning to the website to ensure they aren’t swimming in the same gene pool.
On Íslendingabók, Mr. Theodórsson discovered he and the woman from Tinder had the same great-grandfather.
“We decided to not speak of this again and try to avoid each other at the next clan meeting,” Mr. Theodórsson said.
Previously Mr. Theodórsson had been engaged to a woman related five generations back. That was fine, he said, though it was still uncomfortable when her grandmother and his aunt spoke to each other using terms of endearment reserved for close relatives at the dinner table.
Tracing a person’s lineage in Iceland is especially challenging because last names are no indicator of historic family lineage. In Iceland a person’s last name is usually the father’s first name, followed by “son” or “dottir.”
Jona Holmgeirsdóttir, a 23-year-old student who lives in Reykjavik, Iceland’s largest city of about 120,000, said that after a night out, it was common for her friends to look up people they had met on Íslendingabók.
“We often say, ‘Have you googled him? Have you looked him up in the Book of Icelanders?’” Ms. Holmgeirsdóttir said. “You have to use it.”
A flower-shop owner in Borgarnes by the name of Katrín Huld Bjarnadóttir said she wishes she had access to the Íslendingabók webpage when she was dating.
“Well, we didn’t date…it was just that one time,” Ms. Bjarnadóttir said as her voice trailed off. “This was before all this internet, social media, everything.”
“We found out on the same day,” she said. Her grandfather and his grandmother were siblings. “He called me and my mom had just told me and I was like ‘Yeah, we shouldn’t do this again.’”
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Dr. Kári Stefánsson, CEO and co-founder of deCODE genetics, the Iceland based biopharmaceutical company behind Íslendingabók, says the database isn’t a dating app.
On the 10th anniversary of Íslendingabók’s online debut, an app development competition was held. It led to an app that allowed users to bump their phones together to instantly trace how their family trees collide. “Bump in the app before you bump in bed,” was the tag line coined by the group of software engineering students behind the product.
Dr. Stefánsson emphasizes the distinction between the app and his widely used database. While he says there may be various reasons people consult Íslendingabók, its primary use is to facilitate genetic research. DeCODE genetics focuses on mapping genes associated with diseases using population studies, and applies the knowledge in the development of drugs.
After falling in love with a man named Hrannar Hrannarsson she met at a disco in Reykjavik in 1992, Margrét Sigurjónsdóttir was told later on by a relative that she shared the same great grandparents as her new boyfriend’s mother. Her brother and Mr. Hrannarsson’s aunt were even christened in the same dress. Mrs. Sigurjónsdóttir said that since they began dating before Íslendingabók’s creation, they didn’t know about their shared roots.
“I remember I cried because I thought we would have to split up,” she said.
But they didn’t. Today they are happily married with three healthy children.
Though Mrs. Sigurjónsdóttir, 45, didn’t have Íslendingabók to help navigate her dating years, she logged on to the database shortly after its release out of curiosity. She was surprised yet again.
There was a list of her ancestors a few hundred years back. Included was a column labeled “siblings” where her brothers Alfred and Svavar and her sister Bara were listed.
“Then there was this other column that said ‘half-siblings’ and there was her name, Gudbjorg Danielsdottir, born 1968!” Mrs. Sigurjónsdóttir said. “And beside her name said ‘same father.’ I was speechless.”
Mrs. Sigurjónsdóttir and her siblings met their sister a few days later and gave her a card with a pink stork on it. They wrote, “Welcome to our world little/big sister.”
All children who are registered in the National Registry are automatically added to Íslendingabók. However, people who have been adopted can decide whether their biological parents are shown in their family trees.
Ms. Danielsdottir said: “I decided to write the names of my biological parents as well as my adoptive parents since I think it matters to know where your genes comes from.”
When she got married, Mrs. Sigurjónsdóttir invited her new sister to the wedding and introduced her to both sides of the interconnected families.
Mr. Theodórsson, who dated women in Iceland both before and after the Íslendingabók social-media revolution, maintains that it has changed how he approaches his love life.
“I thought it was kind of a joke before, how likely it would be to accidentally date your cousin,” he said. “And then it happened to me and I was like, ‘Maybe it’s way more common than I thought.’