Stylometry and authorship

1
Stylometry and authorship

• D. Holmes Authorship attribution Computers and
  the Humanities 28 (1994), 87-106.
• D. Holmes The Evolution of Stylometry in
  Humanities Scholarship Literary and Linguistic
  Computing 13 (1998), 111-117. http//llc.oxfordjou
  rnals.org/cgi/reprint/13/3/111.pdf
• T. McEnery M. Oates Authorship identification
  and computational stylometry in Dale et al (eds)
  Handbook of Natural Language Processing, New York
  (2000) Dekker, chapter 23.1

2
Stylometry

• Measurement of (aspects) of style
• Especially using computational tools
• Purposes
• Genre classification
• Historical study of language change (diachronic
  linguistics)
• Literary analysis
• Authorship attribution
• Forensic linguistics

3
Authorship attribution

• Has been a topic of research since at least
  mod-19th century (predates computers)
• Interest in
• resolving issues of disputed authorship
• identifying authorship of anonymous texts
• may be useful in detecting plagiarism, and
  authorship of computer viruses
• used in forensic setting, eg to detect genuine
  confessions

4
Authorship attribution

• Five main approaches
• Physical evidence
• eg carbon dating and handwriting analysis, as in
  case of Hitler Diaries. Not relevant to
  linguistics/stylistics
• Historical evidence
• eg did Marlowe or Shakespeare write Edward III?
  It was published 1596, 3 yrs after Marlowes
  death, but contains references to the defeat of
  the Armada (1588)
• knowledge intensive, not feasible for computers

5
Authorship attribution

• Cipher-based decryption
• idea that authors deliberately encode their names
  in text
• especially widespread in Bible studies, but also
  in Shakespeare-Bacon debate
• Penn (1987) used computer analysis to show Bacon
  had written a lot of Shakespeares plays
• easily debunked see http//shakespeareauthorship.
  com/5b Ross showed that using the same
  techniques proved that bacon also wrote
  Spensers Faerie Queene, the Bible, Caesars
  Gallic Wars, Hiawatha, Moby Dick and The
  Federalist Papers (see later)

6
Authorship attribution

• Manual analysis
• Much used in forensic linguistics
• Detailed analysis of unlimited linguistic traits
• Not suitable for computational analysis, but
  well look at some examples later
• Computational stylometry
• Involves counting things
• So can only look at what is easily countable

7
Stylometry

• Assumes that the essence of the individual style
  of an author can be captured with reference to a
  number of quantitative criteria, called
  discriminators
• Obviously, some (many) aspects of style are
  conscious and deliberate
• as such they can be easily imitated and indeed
  often are
• many famous pastiches, either humorous or as a
  sort of homage
• Computational stylometry is focused on
  subconscious elements of style less easy to
  imitate or falsify

8
Stylometry is not foolproof

• We should be aware of shortcomings
• Discriminators are mostly lexical, though some
  recent work has looked also at syntactic
  discriminators
• Authors styles change, either over time, or
  deliberately, eg when writing in different
  literary genres
• Many techniques rely on large quantities of data
• Most of the following techniques are better at
  dealing with closed questions
• Who wrote this, A or B?
• If A wrote these, did they also write this?
• How likely is it that A wrote this?
• but not Who wrote this?

9
Some classical examples

• Did Homer write both the Illiad and the Odyssey?
• both generally attributed to a single individual
  named Homer, but both are derived from long
  oral tradition
• Did Paul write all the NT Letters of St Paul?
• Especially, the authorship of Hebrews has long
  been debated on theological grounds
• Plato developed his philosophy in the form of
  dialogues, putting his own doctrines into the
  mouth of Socrates his teacher.
• Ascertaining the correct chronological order of
  these dialogues would help to understand how
  Plato developed his philosophy
• Did Shakespeare write all of his plays?
• Various authors including Bacon and Marlowe are
  said to have written parts or all of several
  plays
• Shakespeare may even be a nom-de-plume for a
  group of writers
• two more plays Edward III and Two Noble Kinsmen
  may have been written partly by Shakespeare

10
Some modern examples

• The Federalist Papers
• a series of articles published in 1787-88 with
  the aim of promoting the ratification of the new
  US constitution.
• written by three authors, Jay, Hamilton and
  Madison, under the pseudonym Publius
• Some are of known (and in some cases joint)
  authorship but others are disputed
• Pioneering stylometric methods were famously used
  by Mosteller and Wallace in the early 1960s to
  attempt to answer this question
• It is now considered as settled
• The Federalist Papers present a difficult but
  solvable test case, and are seen as a benchmark
  to test new ideas

11
Some modern examples

• Similarities with private letters helped to
  identify the style of the Unabombers manifesto
• Unabomber Theodore Kaczynski perpetrated a number
  of bomb attacks on universities and airlines
  between 1978 and 1995
• Promised to stop if his 35,000-word
  anti-industrialist manifesto was published in
  major newspapers
• Distinctive writing style and turns of phrase
  enabled him to be identified
• Authorship of Primary Colors, a work of fiction
  about preparations for the Democratic primaries
  which showed the Bill Clinton character in a bad
  light

12
Some modern examples

• Derek Bentley and his disputed murder
  confession (1953)
• Bentley (an illiterate man of low IQ) and another
  man involved in an armed robbery in which a
  policeman was shot
• Bentley found guilty and hanged in January 1953
• In 1971 author Yallop looked closely at the case,
  
• As well as conflicting ballistic evidence, and
  some procedurtal errors in the trial, Bentleys
  statement was found to have been doctored by
  police
• Contested statement used then every 58 words on
  average and repeatedly used I then.
• BoE uses then every 500 words, and then I ten
  times more often than I then. Importantly,
  witness statement frequencies overall are similar
  to BoE.
• Police statement genre of the time used then
  every 78 words, and typically used the I then
  form.
• Derek Bentley acquitted in 1999, posthumously,
  appeal assisted by a linguistics professor

13
Basic methodologies

• Word or sentence length too obvious and easy to
  manipulate
• Frequencies of letter pairs strangely successful,
  though limited
• Distribution of words of a given length (in
  syllables), especially relative frequencies, ie
  length of gaps between words of same syllable
  length.

14
Vocabulary richness

• Based on the idea that authors vocabulary is
  more or less constant
• Various measures
• Type-token ratio
• Simpsons index (the chance that two word
  arbitrarily chosen from text will be the same)
• Yules K (occurrence of a given word is a chance
  occurrence can be modelled as a Poisson
  distribution)
• Entropy (measure of uniformity)

15
The Federalist Papers

• 85 papers arguing for the adoption of the US
  constitution
• written by three authors (Jay, Hamilton, Madison)
• 5 authored by Jay
• 51 authored by Hamilton
• 14 authored by Madison
•   3 jointly by Hamilton and Madison
• authorship of 12 of them disputed (Hamilton or
  Madison?)
• Mosteller and Wallace (1964) employed function
  words such as prepositions, conjunctions, and
  articles as discriminators.
• e.g., the word upon averaged 3.24 appearances per
  1,000 words in the known writings of Hamilton but
  only 0.23 in the writings of Madison
• 30 marker words identified as discriminativeof
  the two contested authors upon, whilst, there,
  on, while, vigor, by, consequently, would, voice

16
Bayesian probability

• Bayes hypothesis reconciles prior hypotheses (in
  this case based on historical observation) with
  conditional probabilities based on measurements
• If prior hypothesis (eg that there is a 13
  chance that Madison wrote the paper) is confirmed
  by the measurements (eg of features associated
  with Madisons style), the result will be neutral
• If prior hypothesis is contradicted by the
  measurements, result will be much more striking

17
Cumulative sum charts

• Method
• Assume authorial fingerprints such as
  percentage of short words, or words beginning
  with a vowel
• Put two texts together and plot the number of
  items per sentence against the cumulative average
• If graph has a sharp divergence at the point
  where the texts are joined, this shows the
  authors differ
• Highly controversial
• Interpretation of graphs very subjective
• But much used in courts!
• Weighted cusum
• Slightly sounder footing statistically
  eliminates need for subjective judgment
• Still not very accurate compared to other measures

18
Multivariate analysis

• Thanks to computers it is now possible to collect
  large numbers of different measurements, of a
  variety of features
• Variants of multivariate analysis
• Cluster analysis
• Correspondence analysis
• Principal components analysis

19
Cluster analysis

• Group objects according to their similarity with
  respect to a given feature
• Produces a tree diagram or dendogram

20
Correspondence analysis

• Example of superlatives in Dickens and
  Smolletts works
• Tabata 2007 http//www.digitalhumanities.org/dh20
  07/abstracts/xhtml.xq?id259)
• Count frequency of 242 superlatives in 30 texts
• CA allows classification of associations between
  variables in a 2d matrix, rows x columns
• D1 distinguishes Dickens from Smollett
• D2

21
Principal components analysis

• Like cluster analysis but can work with much
  larger range of variables
• PCA is a statistical method for arranging large
  arrays of data into interpretable patterning
  match
• principal components are computed by
  calculating the correlations between all the
  variables, then grouping them into sets that show
  the most correspondence
• each set is a component, or dimension

22
Final word

• Many of these techniques are also used to
  identify different genres rather than different
  authors
• especially PCA, where the dimensions can be
  characterised
• (In fact, cluster analysis and PCA illustrations
  were taken from such a study!)
• An interesting question how well do they work on
  pastiches?
• If interested, see H Somers F Tweedie
  Authorship attribution and pastiche, Computers
  and the Humanities 37 (2003), 407-429.