The base MIDUS twin sample consists of 998 pairs. Zygosity was determined using self-report questions regarding information such as similarity of eye and hair color and degree to which others were confused as to their identity during childhood. Such techniques are generally more than 90% accurate (Lykken, Bouchard, McGue, & Tellegen, 1990); however, 16 pairs in this sample were not considered classifiable due to missing or indeterminate zygosity information. We also excluded 263 opposite-sex pairs from the full MIDUS twin sample, resulting in a sample of 719 pairs. Of the base sample of 1438 respondents (719 pairs), 69.75% were married, comparable to the percentage of married persons in the MIDUS main (non-twin) sample (see Marks, Bumpass, & Hun, 2004). We studied twin pairs who were concordant for being married at the time of data collection because marital quality was the focus of this research. This eliminated 435 individual respondents who were not married. We then eliminated 254 individuals for whom co-twin data was not available because of discordant marital status. This resulted in a final sample of 379 twin pairs:
