He studied medicine at the University of Valladolid and joined the army’s sanitary corps in 1910, taking part in the Rif War between 1912 and 1915.

He advocated eugamia, a eugenic policy implemented through premarital orientation work based on the biopsychological assessment of a couple’s personality.

Vallejo Nájera carried out experiments on female Spanish Republican Army prisoners in Málaga and International Brigades members held in the monastery of San Pedro de Cardeña[2] to establish "the bio-psychic roots of Marxism" and find the "red gene".

Some will live in permanent exile... Others will lose their freedom, groaning for years in prisons, purging their crimes with forced work in order to earn their daily bread...".

[12] According to Paul Preston the "investigations" of Vallejo-Najera provided the Francoist State with "scientific" arguments in order to "justify their views on the subhuman nature of their adversaries".

[4] When the Civil War ended, Vallejo's support for Francoism was rewarded by his appointment as Professor of Psychiatry at the University of Madrid.