According to Sigmund Freud's The Interpretation of Dreams, falling dreams fall under the category of "typical dreams", meaning the "dreams which almost everyone has dreamt alike and which we are accustomed to assume must have the same meaning for everyone".[13] In the fairly recent study, "The Typical Dreams of Canadian University Students", common dreams were investigated by administering a Typical Dreams Questionnaire (TDQ).[14] The results confirmed that typical dreams are consistent over time, region, and gender, and a few themes can be considered almost universal: falling (73.8% prevalence), flying or soaring in the air (48.3%) and swimming (34.3%). In 1967, Saul and Curtis published a paper entitled "Dream Form and Strength of Impulse in Dreams of Falling and Other Dreams of Descent".[15] According to Saul and Curtis, dreams of falling can have various meanings, such as the sensation of falling asleep, the symbolization of a real risk of falling from bed, the repetition of traumatic experiences of falling or sensations of falling from parents’ arms in childhood, birth and delivery, ambition or the renouncement of responsibility, or life experiences such as flying in an airplane. They quote another author, Gutheil (1951), who suggests a range of possible meanings subsumed under the general idea of loss of (mental) equilibrium.