Eye colour is mainly determined by genetics and the quantity and type of pigments in the iris. Two main types of pigment responsible for eye colour are brown melanin and blue melanin. 

Brown melanin is responsible for colours ranging from brown to black, while blue melanin is responsible for blue hues. Genetics influence the amount and type of melanin a person inherits from their parents. 

In Scandinavian countries, blue eyes have a relatively high prevalence, which can be partly attributed to genetic inheritance. Scandinavian populations have historically had limited genetic diversity due to factors such as geographical isolation and limited migration. This has contributed to the fixation of certain genetic characteristics, including the predisposition to blue eyes.

Numerous studies claim that blue eyes, blond hair and fair skin, typical of Nordic countries, are an adaptation to low sunlight, favouring vitamin D absorption. 

However, it is important to note that eye colour is a polygenic trait, meaning that several genes influence it. Consequently, even within Scandinavian populations, there is a diversity of eye colours, although blue eyes are the most common.