The original population of Iceland was of Nordic and Celtic origin. This is evident by literary evidence from the settlement period as well as from later scientific studies such as blood type and genetic analysis. One such genetics study has indicated that the majority of the male settlers were of Nordic origin while the majority of the women were of Celtic origin.
Iceland has extensive genealogical records dating back to the late 1600s and fragmentary records extending back to the Age of Settlement. The biopharmaceutical company deCODE Genetics has funded the creation of a genealogy database which attempts to cover all of Iceland's known inhabitants. It sees the database, called Íslendingabók, as a valuable tool for conducting research on genetic diseases, given the relative isolation of Iceland's population.
The population of the island is believed to have varied from 40,000 to 60,000 in the period from initial settlement until the mid-19th century. During that time, cold winters, ashfall from volcanic eruptions and bubonic plagues adversely affected the population several times. The first census was carried out in 1703 and revealed that the population was then 50,358. After the destructive volcanic eruptions of the Laki volcano during 1783-1784 the population reached a low of about 40,000. Improving living conditions triggered a rapid increase in population since the mid-19th century – from about 60,000 in 1850 to 313,000 in 2007.
Icelandic, the national language, derives from the Old Norse language that was spoken throughout Scandinavia at the time of settlement. It has changed little through the centuries, partly because of the country's isolation and partly because of the people's familiarity with the classical language, as preserved in early historical and literary writings. There is comparatively little difference between the old language and the modern, or between the written language and the spoken. To this day, Icelanders are able to read the great 13th-century sagas without special study.
English is widely spoken, and many Icelanders speak it at an almost native level. Danish is also widely understood. Studying both these languages is a mandatory part of the compulsory school curriculum. Other commonly spoken languages are German, Norwegian and Swedish. Danish is mostly spoken in a way largely comprehensible to Swedes and Norwegians – it is often referred to as "Scandinavian" in Iceland.
The Evangelical Lutheran Church, the national church, is endowed by the state, but there is complete freedom for all faiths, without discrimination. All Iceland constitutes a single diocese of the national church, headed by a bishop with his seat at Reykjavík; there are 281 parishes.
As of 2002, about 87% are nominally members of this established church, though it is believed that most do not practice actively. About 4% belong to one of three Lutheran Free Churches: The Reykjavík Free Church, the Hafnarfjordur Free Church, or the Reykjavík Independent Church. Another 4% (about 11,471 people) belong to one of 20 different denominations that are registered and recognised by the state. The largest of these groups are the Roman Catholics (4,803 members) and the Pentecostal Church (1,630 members), Seventh-Day Adventists (725 members), and Jehovah's Witnesses, (638 members). The Buddhist Association of Iceland has about 445 members, the Baha'i Community has about 387 members, and the Muslim Association has about 178 members.
Other groups are primarily Christina-based organisations. Judaism is practiced by some citizens, however, there have been no requests for official recognition from the Jewish community.