Personality is composed of traits an individual acquires, dimensions of character; and traits with biological underpinnings, dimensions of temperament. Many traits of temperament are heritable and linked to specific gene pathways and/or hormone or neurotransmitter systems. Academic literature associates specific constellations of temperament traits with four broad yet specific human neural systems, the dopamine (DA)/norepinephrine(NE) system, and the serotonin (5HT), testosterone(T) and estrogen(E)/oxytocin(OT) systems. For example, activities in the DA/NE system are associated with risk taking, novelty seeking, boredom susceptibility and disinhibition, as well as energy, enthusiasm, physical and intellectual exploration, cognitive flexibility, curiosity, idea generation and verbal and non-linguistic creativity. Alleles of the serotonin system are associated with sociability and extroversion, as well as religiosity, conformity, orderliness, conscientiousness, concrete thinking, self-control, sustained attention, low novelty seeking, and figural and numeric creativity. Traits linked with testosterone expression are heightened attention to detail, intensified focus, emotional containment, emotional flooding (particularly rage), aggressiveness, less social sensitivity, and heightened spatial and mathematical acuity. Verbal fluency and other language skills are linked with estrogen activity. Empathy, nurturing, the drive to make social attachments, and other prosocial skills are associated with estrogen and oxytocin. Estrogen activity is also associated with contextual thinking, imagination and mental flexibility. This paper discusses the traits associated with these four biological systems, a new measure of these four temperament dimensions (Fisher-Rich-Island-Neurochemical Questionnaire, or FRI-NQ), and data on 1.24 million anonymous men and women collected on the Internet dating site, Chemistry.com, to show how the sexes vary in the expression of these four neural systems. The paper then discusses data collected on 28,128 anonymous members of Chemistry.com to show the mate choice preferences of men and women primarily expressive of each of these four temperament dimensions. Heterosexual men and women primarily expressive of the DA/NE system are initially attracted to one another, as are heterosexual men and women primarily expressive of 5HT. But heterosexual men and women primarily expressive of T were initially attracted to those primarily expressive of E/OT, and heterosexual men and women primarily expressive of E/OT were initially attracted to those primarily expressive of T. The sex steroids, testosterone and estrogen, appear to play a role in temperament and mate choice.