He headed the Boston Psychopathic Hospital when it opened in 1912, pioneering the study of brain pathology with particular interests in shell shock and schizophrenia.

He was president of the American Medico-Psychological Association and the Boston Society of Psychiatry and Neurology, and held advisory positions with the U.S. Chemical Warfare Service and the Eugenics Record Office.

His interest in chess continued throughout his life, and he enjoyed intellectual gatherings at the home of art collector and friend Walter Arensberg.

His mother was a schoolteacher for several years; his father, who supervised a cotton-waste factory and established a trucking business, earned enough money to ensure that Southard did not have to work during his undergraduate and graduate studies.

[3] Southard attended Boston Latin School, where his father, aunt and headmaster Arthur Irving Fiske sparked a lifelong interest in language and the meaning of words.

He learned about comparative anatomy and the nervous system from biologist George Howard Parker, studied psychology under William James, took a class in logic taught by Josiah Royce[8] and graduated with a degree in philosophy.

[13][14] After returning from Germany, Southard interned in pathology at Boston City Hospital and became an instructor at Harvard Medical School in 1904.

Although he wrote an outline of his autobiography and traveled extensively in Europe during his convalescence, he felt unable to concentrate on research and referred to this period as "the wasted year".

[13] He served in a strategic advisory role with the U.S. Army Chemical Warfare Service during World War I, attaining the rank of major.

He believed that shell shock resulted from the mind's inability to align the sensory experiences of war with other life events.

Once shell shock was no longer thought to result from physical injuries, patients were stigmatized and arguments over its cause interfered with effective treatment.

[26] In 1924, a bronze table by sculptor Bashka Paeff was installed in the reception of the hospital in his honor,[27] Although Southard expressed a great deal of interest in research, he was most inclined to work on the classification, nomenclature and definition of psychiatric and philosophical concepts.

He said he realized that such work was ridiculed by many, but a "psychiatric dictionary (to include definitions of every near-lying psychological and philosophical term also) would do more to push mental hygiene on than any other single thing I can think of.

[28] He was particularly interested in dementia praecox (which he favored renaming schizophrenia), and found diffuse anatomic differences in the brains of schizophrenic patients.

[30] Shortly before his death Southard wrote and presented Non-dementia non-praecox: note on the advantages to mental hygiene of extirpating a term, but did not live to see it published.

[29] Southard and Mary Jarrett founded the field of psychiatric social work, applying psychiatry to industrial employees.

Shortly afterwards, Yerkes was elected president of the American Psychological Association and developed the U.S. Army's mental testing program during World War I.

"[36] In 1906 Southard married Mabel Fletcher Austin, a Wellesley College mental-hygiene lecturer and Johns Hopkins University graduate.

[38] Southard wrote to Frederic Parker Gay about the limitations his professional responsibilities placed on his marriage: "Mabel is her own cook, maid and bath steward, as for her being a wife, I have little or no time to be a husband.

His younger son, Ordway, was an early writer of English-language haiku[40] and published under several names, including O. Mabson Southard, O.M.B.

[43] Southard experienced chronic headaches and minor seizures (sometimes accompanied by partial vision loss for several hours), which he attributed to mental strain.

[44] According to Gay, a physical examination several months before Southard's death may have indicated an endocrine gland problem, but no specific condition was diagnosed.

After his death, in "metaphors more appropriate for a comet than a man",[43] friends described the intellect which allowed Southard to play up to six blind chess matches simultaneously.

[37] Southard frequently traveled from Boston to New York City to participate in Walter Arensberg's salons, bringing scholarship to discussions of contemporary social science topics.

[49] Canavan later wrote that Southard had experienced "singular difficulties producing considerable mental discomfort"[43] during the last year of his life.