To cite package 'umx' in publications use:

Bates TC, Neale MC, Maes HH (2019). “umx: A library for Structural Equation and Twin Modelling in R.” Twin Research and Human Genetics, 22, 27-41. doi:10.1017/thg.2019.2.

Corresponding BibTeX entry:

  @Article{,
    title = {umx: A library for Structural Equation and Twin Modelling
      in R},
    author = {Timothy C. Bates and Michael C. Neale and Hermine H.
      Maes},
    journal = {Twin Research and Human Genetics},
    publisher = {Cambridge Press},
    year = {2019},
    volume = {22},
    pages = {27-41},
    doi = {10.1017/thg.2019.2},
  }