Aim - determine heritability of MDD in a large swedish sample
Kendler et al 2006
Participants - Over 15000 complete twin pairs registered in the national SwedishTwinRegistry
Kendler et al 2006
Procedure - They carried out telephoneinterviews over a period of 4 years to diagnose lifetimeMDD on the basis of the DSM-IV symptoms or having had a prescription for antidepressants. The twins were also asked about their shared or individual specific environment that may have made a twin more susceptible to depression
Kendler et al 2006
Results - The average concordancerate for MDD across the twins was 38%. There was no correlation between number of years twins had lived together and MDD, suggesting it was a true heritability rate. The rate among female MZ twins was higher than among male MZ twins. Rates for dizygotic twins were lower overall. The difference between MZ and DZ concordancerates is enough to indicate a strong genetic component
Kendler et all 2006 was correlational so no cause and effect. There was no particulargeneisolated, but it did confirm previous research about reliability. Information about lifeevents and MDD symptoms were selfreported.Diagnosis by telephone may be of questionablevalidity.Large sample size mediates some of the concerns above though