Does quality of marital sex decline with duration?

Does the quality of marital sex increase or decrease with marital duration? Previous research assumes that it decreases; however, there is no empirical evidence of declining quality of marital sex with duration in the literature. This study theoretically and empirically examines how the quality of marital sex changes with duration. Theoretically, two effects may influence the change of quality of marital sex: the effect of diminishing marginal utility (the marginal utility of consuming a good or service diminishes as the consumption of that good or service increases) and the effect of the investment in the marriage-specific human capital (including the "partner specific" skills that enhance the enjoyment of marital sex and the knowledge about the spouse's sexual preferences, desires, and habits). The quality of marital sex could either increase or decrease depending on which effect is dominant. The multivariate analysis of the National Health and Social Life Survey data shows that marital duration has a small and negative effect on the quality of marital sex. The gender difference in the quality of marital sex is discussed.