A twin and adoption study of persistence

Introduction

Some people keep working on a hard problem long after others give up. Researchers asked how much of this difference in persistence is linked to heredity and how much to the environment people grow up in. They hypothesized that persistence is partly heritable: pairs of people who share more genes would have more similar persistence scores, even when raised apart.

Participants

1,240 adults from a national twin and adoption registry in one Northern European country. They formed five kinds of pairs: 210 pairs of identical twins raised together; 190 pairs of fraternal twins raised together; 60 pairs of identical twins raised in different adoptive families; 85 pairs of adopted siblings (not related by birth) raised in the same home; and 75 pairs of adoptees and their birth parents. Everyone except the birth parents was aged 18 to 30; the birth parents were 38 to 62. 58% were women. Almost all had finished secondary school.

Method

Each participant worked alone at a computer on a set of puzzles. Unknown to them, the last puzzle had no solution. Persistence was the number of minutes a participant kept working on that puzzle before pressing "give up" (the program stopped at 30 minutes). Researchers then computed a correlation between the two members of each kind of pair. The study was approved by a university ethics review board. Participants gave written informed consent, were told afterward that the last puzzle could not be solved, and were identified only by code numbers so that no one outside the research team could link scores to names.

Results

Identical twins raised together had the most similar scores ($r = .50$), followed by identical twins raised apart ($r = .44$). Fraternal twins raised together were less similar ($r = .26$). Adoptees and their birth parents showed a small correlation ($r = .21$), while adopted siblings raised in the same home showed almost none ($r = .06$).

Discussion

Pairs who share more genes had more similar persistence, even when they grew up in different homes, which suggests heredity contributes to persistence. Because no correlation was close to 1, the environment and chance also matter. The authors caution that a twin study cannot identify which genes, or which experiences, are involved.

Invented study for practice.