Loneliness is an adverse emotional reaction thought to stem from an unwanted and impoverished social situation. Though it commonly makes brief appearances across the lifespan for most people, it has received increasing attention as a factor relevant to somatic and psychological well-being when assuming a more chronic form. For this reason, developing ways of alleviating loneliness is an important item on the research agenda tied to this phenomenon. Psychological interventions, and cognitive behavioural therapy (CBT) in particular, have been proposed to have potential for this. This thesis sought to evaluate the effects of two different kinds of internet-based interventions targeting loneliness: one based on CBT, and one based on interpersonal psychotherapy (IPT).

In addition to this general aim, Study I also investigated the presence of different subgroups in the sample of people seeking help within the framework of projects. Using the statistical method known as Latent Profile Analysis we discovered five profiles consisting of symptoms of common psychiatric disorders and loneliness. The profiles mainly differed as a function of symptom severity, though one of the larger groups was also characterised primarily by their high ratings of social anxiety. The results suggest that the sample seeking help for their loneliness can exhibit both clinical and non-clinical levels of common mental health problems.

Study II served as the pilot evaluation of an ICBT programme for loneliness. A total of 73 participants were included in a randomised controlled trial where the participants were randomised to either 8 weeks of active treatment or a wait-list control group. The results indicated significantly lower loneliness ratings after the treatment phase for the ICBT condition with a moderate-to-large effect size compared to the control group. Significant differences favouring the ICBT condition were also noted for two of the four secondary measures.

Study III followed up on the participants two years after the conclusion of the initial treatment period. At this point, the control group had also received access to a version of the ICBT programme with therapist support on-demand. The results indicated that the decrease in loneliness was sustained, along with similarly lasting effects on the secondary outcomes of interest.

Study IV aimed to replicate the findings from the second study with a similar ICBT programme. However, this study also employed an internet-based IPT intervention to allow for conclusions regarding the possibility of reducing loneliness by other means than CBT. A sample of 170 participants were recruited and randomised to one the treatment conditions or to a waitlist control group. The results indicated that the ICBT condition had a significantly steeper reduction in loneliness than both the waitlist and the IPT condition after the conclusion of the treatment. Both active conditions produced a significant increase in quality of life.

In conclusion, internet-based psychological interventions can be efficacious for reducing loneliness, though the efficacy was only found for participants who received access to the ICBT condition in Study II and IV. The benefits from this treatment programme were sustained up to two years after the conclusion of the intervention. For these reasons, ICBT is proposed to be a good candidate for offering help to people experiencing distressing feelings of loneliness.

Ensamhet är en aversiv känsloreaktion som har sitt ursprung i en oönskad social situation. I kronisk form har den påkallat uppmärksamhet som en faktor med implikationer för fysisk och psykisk hälsa. På grund av detta har tillvägagångssätt för att minska ensamhet blivit en viktig punkt på forskningsagendan kopplad till detta fenomen. Psykologiska interventioner, och mer specifikt kognitiv beteendeterapi (KBT), har hävdats ha potential för att åstadkomma detta. Denna avhandling utvärderade effekterna av två internetbaserade interventioner inriktade mot ensamhet: en baserad på KBT och en baserad på interpersonell psykoterapi (IPT). Dessutom intresserade sig en av studierna för den deltagarheterogenitet som fanns i termer av psykiatriska symptom.

Personerna som sökte hjälp inom ramarna för studierna kunde delas in i fem undergrupper som främst skiljde sig åt gällande hur grav symptombörda de uppvisade, men i viss mån även sett till vilken typ av psykiatriska symptom som dominerade. Resultaten från de kliniska prövningarna pekar på att det framtagna KBT-programmet har positiva effekter i form av minskad ensamhet och psykiatrisk samsjuklighet. Effekterna kvarstod även över tid. IPT-programmet var inte lika framgångsrikt.