"Horizon of expectation" refers to the context in which the reader or viewer reads or views a literary work or play. In Susan Glaspell's short play Trifles, readers can have one horizon of expectation if they read it from a modern perspective and another if they read it from the perspective of the time period in which it was written and first performed, that is, 1916.
Readers who read the play from a 2016 perspective may feel shocked and appalled that Mrs. Hale and Mrs. Peters would withhold evidence from a criminal case in order to protect another woman. Readers might believe this shows women as being more emotional and less committed to the laws of society. They might take offense that the play unrealistically portrays women who cannot perform investigative work with the same level of detachment and professionalism that men can show. They might draw the conclusion that Glaspell is sexist in her depiction of women.
On the other hand, readers who read the play with an understanding of the era in which it was written, especially knowing that women did not serve on juries in 1916 in most places in the United States, might find this play to be a noteworthy work of feminism. If readers were aware of the companion short story that Glaspell wrote entitled "A Jury of Her Peers," they might consider the fact that Mrs. Wright was going to be subjected to trial before an all-male jury, one that would most likely not be sympathetic toward the emotional and physical abuse Mrs. Wright had received from her husband. Such readers might also take into account the fact that during the early 20th century, resources to help battered women were not readily available and that Mrs. Wright lived in an isolated world at the mercy of her husband. The mockery and belittling that Mrs. Peters and Mrs. Hale receive from the men in the play would be seen as confirmation that women in that era were generally undervalued and did not receive justice in their daily lives, let alone in a court of law. All these factors might lead a reader with such a context to believe that the women did the right thing to withhold evidence since Mrs. Wright was unlikely to receive a fair trial and these women did not want to heap further injustice on a woman who had suffered so much injustice already. In this context, readers might applaud Glaspell for presenting a portrait of two brave women who act on a higher level of morality than the laws of their country enforce. To these readers, this work would be seen as a call to change society to grant equal rights under the law to women.
Depending on one's horizon of expectation, Trifles can be either a sexist or a feminist work.