 6. Temperament  159
Bouchard & Loehlin, 2001; Eysenck, 1990; 
Riemann et al., 1997; Tellegen et al., 1988). 
In this regard, it is noteworthy that Riemann 
et al. (1997) were able to separate out the 
first two effects from the last by conducting 
combined analyses of self- and peer ratings. 
Using this expanded approach, they found 
that nearly 70% of the systematic variance 
in both N/NE and E/PE was due to genetic 
factors, with only roughly 30% attributable 
to the unshared environment and gene– 
environment interactions. The latter takes 
on particular importance in the case of tem -
perament when conceptualized as a set of 
biologically based propensities to respond to 
the environment in certain ways; that is, it is 
largely through gene–  environment interac -
tions that each individual’s unique personal -
ity develops.
Researchers also have examined how ge -
netic and environmental influences may 
vary as a function of demographic variables 
such as sex and age. We previously reported 
that the data regarding sex were markedly 
inconsistent and, given that Vukasovi ć and 
Bratko (2015) did not find support for gen -
der as a moderator of personality heritabil -
ity in their meta-  analysis, we presume that 
the inconsistencies reflect unsystematic sam -
ple variance in the early heritability studies, 
which tended to have smaller samples than 
those collected later.
The evidence regarding age is much more 
systematic, as several studies now have 
found that heritability estimates for both N/
NE and E/PE are significantly lower in older 
respondents (McCartney, Harris, & Ber -
nieri, 1990; McGue et al., 1993; Pedersen, 
1993; Viken, Rose, Kaprio, & Koskenvuo, 
1994). The causes of these age-  related de -
clines have not yet been clearly established, 
but it appears that they may differ across 
these two dimensions. Specifically, McGue 
et al. (1993) found that the lower value for 
N/NE was due to a true decline in heritabil -
ity, whereas that for E/PE was attributable 
to the increased influence of the unshared 
environment (coupled with a stable genetic 
component).
Finally, the recent explosion of genetics 
research has yielded valuable data regarding 
the interesting issue of assortative mating, 
that is, whether or not people partner with 
individuals who are phenotypically (and, therefore, genotypically) similar to them -
selves. In other words, do “Birds of a feather 
flock together” or do “Opposites attract”? 
Assortative mating is of keen interest to be -
havior geneticists because if it occurs, people 
actually will be more genetically similar to 
their first-  degree relatives than the 50% that 
traditionally is assumed in the classic mod -
els. Generally speaking, however, there ap -
pears to be little or no assortative mating 
on the Big Three dimensions (e.g., Beer et 
al., 1998; Eysenck, 1990; Finkel & McGue, 
1997; Watson et al., 2004). Based on a re -
view of earlier literature, Eysenck (1990, 
p. 252) concluded that “mating is essentially 
random for personality differences,” and in 
a recent comprehensive review, Luo (2017) 
supports this conclusion, stating “similarity 
correlations on personality variables tend 
to be positive though with weak magnitude 
rarely above .30” (p. 3 of online PDF). It is 
unclear, however, whether this is because 
temperament is irrelevant in mating or be -
cause both axioms are true to some extent 
and their effects cancel out.
In contrast to the temperament data, Luo 
(2017) presented evidence supporting strong 
similarity coefficients between partners for 
more attitudinal aspects of personality, age 
and socioeconomic status, particularly edu -
cational level, and moderate similarities for 
values, abilities and intelligence, psychopa -
thology, habits and lifestyles (e.g., alcohol 
and caffeine consumption, smoking, exer -
cise, diet), physical and physiological char -
acteristics (e.g., height/weight/shape, blood 
pressure, attractiveness, and health in gen -
eral), but further analyses of these variables 
is beyond our scope.
Finally, with the mapping of the human 
genome and the development of techniques 
that permit widescale study of individuals’ 
genes, new types of research such as linkage, 
candidate gene association, and genomewide 
association studies, plus molecular genetics 
and polygenic analyses. have opened up new 
avenues for studying personality–  gene rela -
tions. Dashing naive hopes that single genes 
would map cleanly onto specific personality 
traits, a major finding of these studies is that 
the pathways from genes to the behaviors 
that collectively constitute personality are 
mind-  bogglingly complex. Although specific 
genes that map onto higher-  order traits have 