Abstract: I examined spousal similarity in personality using 4-waves of data over a 12-year period in the German Socio-Economic Panel. There is very little spousal similarity in actual personality traits like the Big Five. However, there is a high similarity in the halo rating bias between spouses.

Spousal similarity in personality is an interesting topic for several reasons. First, there are conflicting folk ideas about spousal similarity. One saying assumes that “birds of the same feather flock together;” another says that “opposites attract.” Second, there is large interest in the characteristics people find attractive in a mate. Do extraverts find other extraverts more attractive? Would assertive (low agreeableness) individuals prefer a mate who is as assertive as they are or rather somebody who is submissive (high agreeableness)? Third, we might wonder whether spouses become more similar to each other over time. Finally, twin studies of heritability make the assumption that mating is random; an assumption that can be questionable.

Given so many reasons to study spousal similarity in personality, it is surprising how little attention this topic has received. A literature search retrieved only a few articles with few citations: Watson, Beer, McDade-Montez (2014) [20 citations], Humbad, Donnellan, Iacono McGue, & Burt (2010) [30 citations], Rammstedt & Schupp (2008) [25 citations]. One possible explanation for this lack of interest could be that spouses are not similar in personality traits. It is well-known that psychology has a bias against null-results; that is, the lack of statistical relationships. Another possibility is that spousal similarity is small and difficult to detect in small convenience samples that are typical in psychology. In support of the latter explanation, two of the three studies had large samples and did report spousal similarity in personality.

Humbad et al. (2010) found rather small correlations between husbands’ and wives’ personality scores in a sample of 1,296 married couples. With the exception of traditionalism, r = .49, all correlations were below r = .2, and the median correlation was r = .11. They also found that spousal similarity did not change over time, suggesting that the little similarity there is can be attributed to assortative mating (marrying somebody with similar traits).

Rammstedt and Schupp (2008) used data from the German Socio-Economic Panel (SOEP), an annual survey of representative household samples. In 2005, the SOEP included for the first time a short 15-item measure of the Big Five personality traits. The sample included 6,909 couples. This study produced several correlations greater than r = .2, for agreeableness, r = .25, conscientiousness, r = .31, and openness, r = .33. The lowest correlation was obtained for extraversion, r = .10. A cross-sectional analysis with length of marriage showed that spousal similarity was higher for couples who were married longer. For example, spousal similarity for openness increased from r = .26 for newlyweds (less than 5 years of marriage) to r = .47 for couples married more than 40 years.

A decade later it is possible to build on Rammstedt and Schupp’s results because the SOEP has collected three more waves with personality assessments in 2009, 2013, and 2017. This makes it possible to examine spousal similarity over time and to separate spousal similarity in stable dispositions (traits) and in deviations from the typical level (states).

I start with simple correlations, separately for each of the four waves using all couples that were available at a specific wave. The most notable observation is that the correlations do not increase over time. In fact, they even show a slight trend to decrease. This provides strong evidence that spouses are not becoming more similar to each other over time. An introvert who marries an extravert does not become more extraverted as a result or vice versa.

Trait	W1 (N = 6263)	W2 (N = 5905)	W3 (N = 5404)	W4 (N = 7805)
Neuroticism	.16	.11	.08	.09
Extraversion	.10	.07	.03	.07
Openness	.30	.25	.19	.21
Agreeableness	.25	.23	.17	.22
Conscientiousness	.31	.28	.26	.26
Average	.22	0.19	0.15	0.17

I repeated the analysis using only couples who stayed together and participated in all four waves. The sample size for this analysis was N = 1,860.

Trait	W1	W2	W3	W4
Neuroticism	.14	.17	.12	.11
Extraversion	.04	.02	.01	.03
Openness	.23	.23	.19	.20
Agreeableness	.20	.23	.19	.21
Conscientiousness	.29	.24	.29	.23
Average	.18	.18	.16	.16

The correlations were not stronger and did not increase over time.

The next analysis examined correlations over time. If spousal similarity is driven by assortment on some stable trait, husbands’ scores in 2005 should still be correlated with wives’ scores in 2017 and vice versa. To ensure comparability for different time lags, I only used couples who stayed in the survey for all four waves (N = 1,860).

Trait	2005 Trait	2009 Trait	2013 Trait	2017 Trait
2005 Neuroticism	.14	.10	.10	.08
2005 Extraversion	.04	0	-.02	-.02
2005 Openness	.23	.15	.13	.12
2005 Agreeableness	.20	.15	.15	.12
2005 Conscientiousness	.29	.19	.17	.15
Average	.18	.12	.10	.09

The results show more similarity on the same occasion (2005/2005) than across time. Across-time correlations are all below .2 and are decreasing. However, there are some small correlations of r = .1 for Openness, Agreeableness, and Conscientiousness, suggesting some spousal similarity in the stable trait variance. Another question is why spouses show similarity in the changing state variance.

There are two possible explanations for spousal similarity in personality state variance. One explanation is that spouses’ personality really changes in sync, just like their well-being changes in the same direction over time (Schimmack & Lucas, 2010). Another explanation is that spouses’ self-ratings are influenced by rating biases and that these rating biases are correlated (Anusic et al., 2009). To test these alternative hypotheses, I fitted a measurement model to the Big Five scales that distinguishes halo bias in personality ratings from actual variance in personality. I did this for the first and the last wave (2005, 2017) to separate similarity in the stable trait variance from similarity in state variance.

The key finding is that there is high spousal similarity in halo bias. Some couples are more likely to exaggerate their positive qualities than others. After removing this bias, there is relatively little spousal similarity for the actual trait variance.

Factor	Trait	State 2005	State 2017
Neuroticism	.09	.09	.05
Extraversion	-.10	.05	.09
Openness	.13	.12	.15
Agreeableness	.11	.10	.12
Conscientiousness	.14	.22	.16
Halo	.71	.62	.28

In conclusion, spouses are not very similar in their personality traits. This may explain why this topic has received so little attention in the scientific literature. Null-results are often considered uninteresting. However, these findings do raise some questions. Why don’t extraverts marry extraverts or why don’t conscientious people not marry conscientious people. Wouldn’t they be happier with somebody who is similar in their personality? Research with the SOEP data suggests that that is also not the case. Maybe the Big Five traits are not as important for marital satisfaction as we think. Maybe other traits are more important. Clearly, human mating is not random, but it is also not based on matching personality traits.