Atomic Habits: Tiny Changes, Remarkable Results

Chewing gum had been sold commercially throughout the 1800s
: Mary Bellis, “How We Have
Bubble Gum Today,” ThoughtCo, October 16, 2017, 
https://www.thoughtco.com/history-of-
bubble-and-chewing-gum-1991856
.
Wrigley revolutionized the industry
: Jennifer P. Mathews, Chicle: The Chewing Gum of the
Americas, from the Ancient Maya to William Wrigley (Tucson: University of Arizona Press,
2009), 44–46.
Wrigley became the largest chewing gum company
: “William Wrigley, Jr.,” Encyclopædia
Britannica, 
https://www.britannica.com/biography/William-Wrigley-Jr
, accessed June 8,
2018.
Toothpaste had a similar trajectory
: Charles Duhigg, The Power of Habit: Why We Do What We
Do in Life and Business (New York: Random House, 2014), chap. 2.

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he started avoiding her
: Sparkly_alpaca, “What Are the Coolest Psychology Tricks That You Know
or Have Used?” Reddit, November 11, 2016,
https://www.reddit.com/r/AskReddit/comments/5cgqbj/what_are_the_coolest_psychology_tri
cks_that_you/d9wcqsr/
.
The earliest remains of modern humans
: Ian Mcdougall, Francis H. Brown, and John G. Fleagle,
“Stratigraphic Placement and Age of Modern Humans from Kibish, Ethiopia,” Nature 433,
no. 7027 (2005), doi:10.1038/nature03258.
the neocortex . . . was roughly the same
: Some research indicates that the size of the human brain
reached modern proportions around three hundred thousand years ago. Evolution never stops,
of course, and the shape of the structure appears to have continued to evolve in meaningful
ways until it reached both modern size and shape sometime between one hundred thousand
and thirty-five thousand years ago. Simon Neubauer, Jean-Jacques Hublin, and Philipp Gunz,
“The Evolution of Modern Human Brain Shape,” Science Advances 4, no. 1 (2018):
eaao5961.