To cite psychotree in publications use the references for the relevant
functions/data sets:

If you use Bradley-Terry trees (bttree) please cite:

  Strobl C, Wickelmaier F, Zeileis A (2011). “Accounting for Individual
  Differences in Bradley-Terry Models by Means of Recursive
  Partitioning.” _Journal of Educational and Behavioral Statistics_,
  *36*(2), 135-153. doi:10.3102/1076998609359791
  <https://doi.org/10.3102/1076998609359791>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Accounting for Individual Differences in {B}radley-{T}erry Models by Means of Recursive Partitioning},
    author = {Carolin Strobl and Florian Wickelmaier and Achim Zeileis},
    journal = {Journal of Educational and Behavioral Statistics},
    year = {2011},
    volume = {36},
    number = {2},
    pages = {135--153},
    doi = {10.3102/1076998609359791},
  }

If you use Rasch trees (raschtree) please cite:

  Strobl C, Kopf J, Zeileis A (2015). “Rasch Trees: A New Method for
  Detecting Differential Item Functioning in the Rasch Model.”
  _Psychometrika_, *80*(2), 289-316. doi:10.1007/s11336-013-9388-3
  <https://doi.org/10.1007/s11336-013-9388-3>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Rasch Trees: A New Method for Detecting Differential Item Functioning in the {R}asch Model},
    author = {Carolin Strobl and Julia Kopf and Achim Zeileis},
    journal = {Psychometrika},
    year = {2015},
    volume = {80},
    number = {2},
    pages = {289--316},
    doi = {10.1007/s11336-013-9388-3},
  }

If you use rating scale or partial credit trees (rstree/pctree) please
cite:

  Komboz B, Zeileis A, Strobl C (2018). “Tree-Based Global Model Tests
  for Polytomous Rasch Models.” _Educational and Psychological
  Measurement_, *78*(1), 128-166. doi:10.1177/0013164416664394
  <https://doi.org/10.1177/0013164416664394>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Tree-Based Global Model Tests for Polytomous {R}asch Models},
    author = {Basil Komboz and Achim Zeileis and Carolin Strobl},
    journal = {Educational and Psychological Measurement},
    year = {2018},
    volume = {78},
    number = {1},
    pages = {128--166},
    doi = {10.1177/0013164416664394},
  }

If you use MPT trees (mpttree) please cite:

  Wickelmaier F, Zeileis A (2018). “Using Recursive Partitioning to
  Account for Parameter Heterogeneity in Multinomial Processing Tree
  Models.” _Behavior Research Methods_, *50*(3), 1217-1233.
  doi:10.3758/s13428-017-0937-z
  <https://doi.org/10.3758/s13428-017-0937-z>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Using Recursive Partitioning to Account for Parameter Heterogeneity in Multinomial Processing Tree Models},
    author = {Florian Wickelmaier and Achim Zeileis},
    journal = {Behavior Research Methods},
    year = {2018},
    volume = {50},
    number = {3},
    pages = {1217--1233},
    doi = {10.3758/s13428-017-0937-z},
  }

If you use the SPIEGEL Studentenpisa data subsample (SPISA) please
cite:

  Trepte S, Verbeet M (eds.) (2010). _Allgemeinbildung in Deutschland -
  Erkenntnisse aus dem SPIEGEL Studentenpisa-Test_. VS Verlag,
  Wiesbaden. doi:10.1007/978-3-531-92543-1
  <https://doi.org/10.1007/978-3-531-92543-1>.

A BibTeX entry for LaTeX users is

  @Book{,
    title = {{A}llgemeinbildung in {D}eutschland -- Erkenntnisse aus dem {SPIEGEL} {S}tudentenpisa-{T}est},
    editor = {Sabine Trepte and Markus Verbeet},
    year = {2010},
    publisher = {VS Verlag},
    address = {Wiesbaden},
    doi = {10.1007/978-3-531-92543-1},
  }