While not conclusive, some of you may be surprised to discover that there exists some evidence, for a physiological basis for the E/I spectrum.

An fMRI study of personality influences on brain reactivity to emotional stimuli. http://spl.stanford.edu/pdfs/2001%20Behavioral%20Neuroscienc...

Neuroanatomical Correlates of Extraversion and Neuroticism (Neuroticism correlates with introversion in this context) http://cercor.oxfordjournals.org/content/16/12/1809.full

Happy faces trigger different brain reactions in extroverts and introverts http://news.stanford.edu/news/2002/july10/sciencegab-710.htm...

and study: http://www.cogsci.msu.edu/DSS/2012-2013/Gabrieli/CanliScienc...

An event-related potential analysis of extraversion and individual differences in cognitive processing speed and response execution. http://www.ncbi.nlm.nih.gov/pubmed/10821201

Individual Differences in Amygdala-Medial Prefrontal Anatomy Link Negative Affect, Impaired Social Functioning, and Polygenic Depression Risk http://www.jneurosci.org/content/32/50/18087.full