Marital satisfaction was not associated with sexual frequency — not at the start of the marriage, or four years later. Highly satisfied marriages sometimes had high levels of sexual activity, and sometimes low levels — sexual contact alone was not a good indicator of marital satisfaction.

But Russell and McNulty found one important exception. For spouses with high levels of neuroticism, frequent sexual intercourse improved their marital satisfaction. The effect of frequent sexual activity was enough to completely wipe away the “happiness deficit” that neurotic spouses usually have. “Frequent sex is one way that some neurotic people are able to maintain satisfy relationships,” the authors write. The newlywed period is a time when sexual relations are particularly important, and for some — but not all — frequent sex improves their happiness with the marriage. This happiness-by-sex effect occurred regardless of how strong or happy the marriage was at the beginning of the study — frequent sex protects marital happiness for neurotic newlyweds.