Final answer:
Modernism is the most influential "ism" in the development of modern theatre due to its broad definition and alignment with contemporary visual arts movements. It brought forth a new era of critical perspective in the performing arts, emphasizing authenticity and challenging traditional norms.
Step-by-step explanation:
Of all the "isms," modernism has given rise to the most significant body of modern theatre, mainly because of its broad definition and its seeming alliance with a similar movement in the visual arts. Modernism in the arts, which started around the beginning of the 20th century, marked a period of skepticism directed at established authorities, norms, and middle-class complacency. It formed as a reaction to the Victorian ideals of comfort, security, and rigid social structures. Instead, modernism embraced a spirit of experimentation and a new way of looking at human perspectives and society. Theatrical modernism often brought forth styles that were antithetical to traditional theatrical norms, embracing notions from performance art, conceptual art, and complex understandings of identity.
The movement saw a departure from the clear moral messaging of previous eras and instead sought to reflect the complexities of the human condition, much like the social insights provided by figures such as Charles Darwin, Karl Marx, and Sigmund Freud. Theatrical productions from this period focused on authenticity, ephemeral experiences, and strove to avoid the replication or commodification of the performance, thus ensuring a unique experience for each audience.