He was a member of the National Academy of Sciences, a Guggenheim Fellow, and recipient of the Howard Crosby Warren Medal of the Society of Experimental Psychologists.

Following early studies in chemistry, he enrolled at the Berlin Psychological Institute, serving first as assistant to its director, Wolfgang Köhler, and subsequently conducting research of his own.

[4] He completed the work for a PhD degree in 1934,[A] hurrying because his vulnerable position as a Jew in Nazi Germany had led him to decide on emigration.

Wallach considered himself poorly prepared when he took his oral exams, relating later, "I shall never forget the kindness of [two professors] who, aware of my precarious situation, allowed me to pass.".

For the first six years he did not teach but only conducted research, but in 1942 the demands of the war effort depleted the psychology department faculty, and Wallach (who was ineligible for military service) was appointed as an instructor.

"[11]: ix  His studies span a broad range of psychological topics, including the following: Wallach's doctoral dissertation[12] examined perception of lines moving behind an opening in a masking surface – a phenomenon known as the aperture problem.

If a slanted line moves behind such an aperture, the physical stimulus presented to the eyes will not distinguish whether the movement is horizontal, vertical, or at some other angle.

His results provide evidence against a modular scheme of visual processing, where form, color, and motion are computed in isolation.

If, for example, an observer stares for about a minute at a fixation point in the center of a visual field that is white except for a large black rectangle on the left side, and then (with the rectangle removed) looks at the center of an array of four evenly-spaced squares, symmetrically arranged around the fixation point, the two squares on the left side will appear farther apart than the ones on the right.

[18] In 1949 Wallach, in collaboration with Edwin B. Newman and Mark Rosenzweig, published a seminal[B] paper defining the precedence effect in sound localization.

[28] Furthermore, Wallach's highly simplified experimental setup does not deal with three-dimensional spatial arrangements[29] nor with complex visual fields that include many interacting luminances.

[31] This phenomenon illustrates how the visual system processes displays of dynamically-changing elements so that we perceive a world of rigid objects arranged in space.

[32] Others sought to build theoretical models of the essential conditions for dynamically representing rigid three-dimensional objects using only two dimensions, leading to development of a new field of study: structure from motion, a part of the domain of cognitive science.

[33] Practical applications have included representing the third dimension in computer displays,[34] palmtop devices,[35] and airport security scanners.

The altered perception of depth was temporary: it could easily be unlearned (by watching the cube rotate without the telestereoscope), and the effect dissipated spontaneously after a few minutes, even with if the subjects simply sat with eyes closed during that time.

Subsequently, Wallach and Frey[38] performed similar experiments creating a conflict among different cues that the visual system uses to compute the distance of an object from the observer.

A different set of goggles, simulating the oculomotor cues for distances greater than veridical, yielded the opposite result.

These findings – that exposure to cue-conflict situations modifies the way in which the visual system evaluates cues – represented a definite step away from the Gestalt tradition in which Wallach was trained.

[1][2] From the mid-1960s through the end of his career, Wallach engaged in an extensive study of the mechanisms that underlie the apparent stability of the human visual world despite movements of the head, the eyes, or the whole body.

This showed that a highly accurate compensating process corrects for the image displacement that normally accompanies a head movement, thus yielding an appearance of stability.

Wallach called this process constancy of visual direction (CVD), and he noted with interest that it could be easily modified through perceptual adaptation.

As in the case of depth and distance perception, Wallach's finding that the constancy of visual direction adapts readily when stimulus conditions are altered represented a marked departure from the Gestalt tradition, which focused on innate and unmodifiable processes.

For example, Wallach & Bacon[42] were able to demonstrate that two distinct processes are involved in the constancy of visual direction by showing that they adapt differently.

[48] Because he spent his career actively teaching at a liberal arts college, Wallach's legacy includes his influence on the students and research assistants with whom he worked.

Many of these went on to make their own contributions to psychology: Harris[4] provides a partial list, including John M. Darley, Sheldon Ebenholz, William Epstein, Irwin Feinberg, Charles S. Harris, John C. Hay, Eric G. Heinemann, Richard Held, Julian Hochberg, Lloyd Kaufman, Jean Matter Mandler, Jacob Nachmias, Ulric Neisser, Ann O’Leary, Rose R. Olver, Dean Peabody, Mary C. Potter, Judith L. Rapoport, Robert A. Rescorla, Daniel Riesberg, Lance J. Rips, Irvin Rock, Fred Stollnitz, Davida Y. Teller, Lise Wallach (no relation), Michael Wertheimer, and Carl Zuckerman.

In a retrospective article published in 2002, the Swarthmore College Bulletin described him in this way:Wallach firmly established his reputation for brilliant scholarship and an inspirational, decidedly eccentric style.

[6]Another former student, Charles S. Harris, described a characteristic incident: Once, for example, he strode into our seminar room apparently deep in thought, then turned around and walked back out.