MR Dictionary

Occurs when people choose their partners based on particular characteristics, such as height, intelligence, rather than at random. Single-trait assortative mating is characterised by assortment on a single trait (e.g., taller women tend to partner with taller men; mid-height men tend to mate with shorter women). Cross-trait assortative mating occurs when, for example, men with high values of one trait tend to partner with women with either higher or lower values of another trait (e.g., more educated men tend to partner with taller women). 

Cross-trait assortment on the exposure and outcome leads to a spurious genetic correlation between parents, which renders the exposure and outcome phenotypes genetically correlated in the offspring. In some cases, single-trait assortative mating can lead to the same phenomenon. Such spurious genetic correlation can lead to bias in MR. If available parent’s genetic data can be used to detect, and adjusted for, any bias from assortative mating. 

References

Other terms in 'Sources of bias and limitations in MR':