Scientific American 2021 08.pdf

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AUGUST 2021
SCIENTIFICAMERICAN.COM
A New
Understanding
of Alzheimer’s
The Coolest Stars
in the Universe
The Hype about
Hypersonic Weapons
WHY
ANIMALS
PLAY
The rules of the game
for dogs, apes
and elephants
© 2021 Scientific American
Au g u s t 2 0 2 1
VO L U M E 3 2 5 , N U M B E R 2
48
A S T R O N O MY
NEUROSCIENCE
30 Not Quite Stars
Brown dwarfs straddle the line
between stars and planets, and
they might help solve mysteries
about both.
By Katelyn Allers
N E U R O LO G I C A L D I S E A S E
56 The Stuttering Mind
Research on the genetic and neuro-
logical origins of this speech disor-
der is pointing to new treatments.
By Lydia Denworth
SECURIT Y
38 A New Understanding
of A
lzheimer’s
Immune cells called microglia
have become a promising target
for researchers studying the
neurodegenerative disease.
By Jason Ulrich and
David M. Holtzman
Q UA N T U M C O M P U T I N G
64 Overhyped
Physics dictates that hypersonic
weapons cannot live up to the
grand promises made on their
behalf.
By David Wright and
Cameron Tracy
E P I D E M I O LO G Y
44 Chemistry’s
Quantum Future
Quantum computers will bring
molecular modeling to a new level
of accuracy, reducing researchers’
dependence on serendipity.
By  Jeannette M. Garcia
A N I M A L B E H AV I O R
72 The Year Flu
Disappeared
Public health measures meant
to slow the spread of COVID-19
essentially defeated influenza.
By Katie Peek
GENDER STUDIES
Gallo Images
and
Getty Images
74 The World’s First
Trans Clinic
In Germany, the Institute for Sexual
Research would be a century old
if it hadn’t been destroyed by
the Nazis.
By Brandy Schillace
ON tHE C OVE R
Animal play is not just a leisurely pursuit.
Though fun for the participants, it is also
a means of allowing youngsters to practice
skills they will need to succeed later in life—
such as evading predators, forging alliances and
competing for mates—in a safe environment.
Photograph by todd gustafson.
48 Why Animals Play
Frolicking hones physical fitness
and cognition.
By Caitlin O’Connell
August 2021, ScientificAmerican.com
1
© 2021 scientific American
4 From the Editor
6 Letters
10 Science Agenda
Laws that forbid gender-affirming health care
for trans people are unscientific
.
By the Editors
12 Forum
Pesticides are killing worms, beetles and
other organisms that keep our soils healthy.
By Nathan Donley and Tari Gunstone
10
14 Advances
Visualized letters pulled from the brain into text. Hidden
history in imperfect diamonds. Grass that can clean
a toxic explosive. The key to a potent mosquito repellent.
26 Meter
A naturalist marries for science.
By Jessy Randall
28 The Science of Health
Fewer days on antibiotics may be as good as more.
By Claudia Wallis
80 Recommended
14
Why humans cooperate. How animals perceive
the world. A novel of love and wolf research.
What if a pill could edit human consciousness?
By Amy Brady
82 Observatory
Why studies that can’t be confirmed often
get more citations than studies that can be.
By Naomi Oreskes
83 50, 100 & 150 Years Ago
By Mark Fischetti
84 Graphic Science
Bird counts reveal many rare species
and few common ones.
By Clara Moskowitz, Jen Christiansen
and Liz Wahid
80
Scientific American (ISSN 0036-8733), Volume 325, Number 2, August 2021, published monthly by Scientific American, a division of Springer Nature America, Inc., 1 New York Plaza, Suite 4600, New York, N.Y. 10004-1562.
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Scientific American, August 2021
© 2021 scientific American

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