About 48% of variance in the big five personality traits is genetic in origin according to twin studies Vukasović, but we have similar issues we saw in estimating the heritability of intelligence.
Probably the most important issue is that we haven't yet accounted for measurement error, which twin studies lump into the "unshared environment" category. A meta-analysis found an average test-retest correlation of about 0.8 for big five personality tests Gnambs. This bumps up our heritability estimate to 75%. This still ignores imperfections in the test itself, but it's unclear how we account for that.
It also takes a fairly favorable interpretation of "test-retest" - that is tests within a couple months of each other. The same meta-analysis found the test-retest correlation drops significantly if you re-test after a couple years. If we use those correlations, its believing personality is 100% heritable is consistent with the data.
It's hard to know which correlation we should be using to correct for measurement error problems, but the tl;dr is that simply correcting for measurement error puts the plausible heritability of personality in the 75% - 100% range.
There is little assortative mating regarding personality, so at least the problems from that are minimal.
Some researchers have found the heritability of personality falls with age Briley, but, then again, some haven't Loehlin. If we just plot the 20 studies with the smallest standard errors from the meta-analysis at the top of the page, we see a possible small decrease Vukasović:
todo Takahashi
Vukasović, T., & Bratko, D. (2015). Heritability of personality: A meta-analysis of behavior genetic studies. Psychological bulletin, 141(4), 769. https://doi.org/10.1037/bul0000017
Gnambs, T. (2014). A meta-analysis of dependability coefficients (test–retest reliabilities) for measures of the Big Five. Journal of Research in Personality, 52, 20-28. https://doi.org/10.1016/j.jrp.2014.06.003
Takahashi, Y., Zheng, A., Yamagata, S., & Ando, J. (2021). Genetic and environmental architecture of conscientiousness in adolescence. Scientific reports, 11(1), 1-11. https://doi.org/10.1038/s41598-021-82781-5
Loehlin, J. C., & Martin, N. G. (2001). Age changes in personality traits and their heritabilities during the adult years: Evidence from Australian twin registry samples. Personality and Individual Differences, 30(7), 1147-1160.
Briley, D. A., & Tucker-Drob, E. M. (2014). Genetic and environmental continuity in personality development: a meta-analysis. Psychological bulletin, 140(5), 1303. https://doi.org/10.1037/a0037091
