directly related to autism , to create a gradation of autism intensity .
Environmental influences When researchers speak of environmental inputs to traits , diseases and disorders , they are referring to everything from pollutants in the air to subtle perturbations inside cells to cues from other cells . Finding such causative candidates for autism generally involves epidemiological studies that look for correlations between autism rates in a population and an environmental factor of interest .
These connections aren ’ t easy to locate . In the case of genes , if a study involves enough people , even rare genetic differences that make small contributions to autism can often be plucked from the pile . Not so for environmental influences if their effects are significant but small , says Robinson . Within those epidemiological studies , you have to be able to detect that slight signal and assess its power against the larger , background noise of lots of other variations in the cell , body or outside environment that you might not even be aware of and might not be relevant . “ We don ’ t live in a simple , single-exposure world ,” says Kristen Lyall , an epidemiologist at Drexel University in Philadelphia .
And even when a connection is made , its basis is still just math . That is certainly the first step in evaluating a link between an environmental factor and a condition such as autism : As one thing goes up , does the other follow ? But two things that track together don ’ t necessarily share a biological association . ( One of the silliest examples to illustrate how misleading correlation can be is how tightly the number of people killed by venomous spiders each year tracks with the
number of letters in the winning word of the same year ’ s Scripps National Spelling Bee .)
In the case of genetic studies , gene changes with tiny effects
View this
interactive map created by the team at
Spectrum News . The map displays autism prevalence studies conducted at different times and places around the world . Each dot refers to a study . Clicking on the dots reveals granular information such as the country , sample size , years studied , autism prevalence , age of children , diagnostic criteria and sex ratio .
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