{"id":7016,"date":"2017-11-29T15:50:26","date_gmt":"2017-11-29T14:50:26","guid":{"rendered":"http:\/\/emilkirkegaard.dk\/en\/?p=7016"},"modified":"2017-11-29T15:50:26","modified_gmt":"2017-11-29T14:50:26","slug":"twin-control-studies-really-are-evidence-of-causation-reply-to-jayman","status":"publish","type":"post","link":"https:\/\/emilkirkegaard.dk\/en\/2017\/11\/twin-control-studies-really-are-evidence-of-causation-reply-to-jayman\/","title":{"rendered":"Twin control studies really are evidence of causation: reply to JayMan"},"content":{"rendered":"<p>I found some genetically informative studies on the benefits of marriage:<\/p>\n<ul>\n<li><a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pubmed\/21135331\">Does marriage inhibit antisocial behavior?: An examination of selection vs causation via a longitudinal twin design.<\/a><\/li>\n<li><a href=\"http:\/\/psycnet.apa.org\/record\/2012-28539-001\"> Accounting for the physical and mental health benefits of entry into marriage: A genetically informed study of selection and causation. <\/a><\/li>\n<li><a href=\"http:\/\/www.tandfonline.com\/doi\/abs\/10.1080\/07418825.2011.641577\">Marriage and Involvement in Crime: A Consideration of Reciprocal Effects in a Nationally Representative Sample<\/a><\/li>\n<li><a href=\"http:\/\/onlinelibrary.wiley.com\/doi\/10.1111\/j.1741-3737.2011.00884.x\/full\">Marriage and Desistance From Crime: A Consideration of Gene\u2013Environment Correlation<\/a><\/li>\n<\/ul>\n<p>These studies control for genetic and shared environmental confounding in various ways and generally find some benefits of marriage on crime reduction and mental health\/well-being. The benefits from non-controlled studies are found to be exagerated, of course, because they don&#8217;t control for <a href=\"http:\/\/emilkirkegaard.dk\/en\/?p=6636\">the omnipresent genetic confounding<\/a>. I posted one study on Twitter:<\/p>\n<div class=\"oceanwp-oembed-wrap clr\">\n<blockquote class=\"twitter-tweet\" data-width=\"550\" data-dnt=\"true\">\n<p lang=\"en\" dir=\"ltr\">Does marriage reduce crime? Maybe yes. <a href=\"https:\/\/t.co\/ikbgiEav3X\">https:\/\/t.co\/ikbgiEav3X<\/a> <a href=\"https:\/\/t.co\/TrtEkQMd4y\">pic.twitter.com\/TrtEkQMd4y<\/a><\/p>\n<p>&mdash; Emil O W Kirkegaard (@KirkegaardEmil) <a href=\"https:\/\/twitter.com\/KirkegaardEmil\/status\/935526706638675968?ref_src=twsrc%5Etfw\">November 28, 2017<\/a><\/p><\/blockquote>\n<p><script async src=\"https:\/\/platform.twitter.com\/widgets.js\" charset=\"utf-8\"><\/script><\/div>\n<div class=\"oceanwp-oembed-wrap clr\">\n<blockquote class=\"twitter-tweet\" data-width=\"550\" data-dnt=\"true\">\n<p lang=\"en\" dir=\"ltr\">Be wary of twin controls. They can&#39;t show positives (I&#39;m sure I&#39;ll hear it from <a href=\"https:\/\/twitter.com\/SilverVVulpes?ref_src=twsrc%5Etfw\">@SilverVVulpes<\/a>) <a href=\"https:\/\/t.co\/VIGMd1ozVz\">https:\/\/t.co\/VIGMd1ozVz<\/a><\/p>\n<p>&mdash; JayMan (@JayMan471) <a href=\"https:\/\/twitter.com\/JayMan471\/status\/935658291224596481?ref_src=twsrc%5Etfw\">November 28, 2017<\/a><\/p><\/blockquote>\n<p><script async src=\"https:\/\/platform.twitter.com\/widgets.js\" charset=\"utf-8\"><\/script><\/div>\n<p>JayMan (J) doesn&#8217;t agree. In private, he argued that twin control studies don&#8217;t rule out all confounding. This is true, they fail to rule out a very tiny amount of genetic confounding &#8212; MZ twins are not exactly genetically identical, but they are very close. Furthermore, there is the possibility of non-shared non-genetic confounding.<\/p>\n<p>I wrote:<\/p>\n<blockquote><p><span class=\" UFICommentActorAndBody\"><span data-ft=\"{&quot;tn&quot;:&quot;K&quot;}\"><span class=\"UFICommentBody\">Given that non-shared non-genetic variance is noise, one can indeed infer causation. Within MZ associations are strong evidence of causation.<\/span><\/span><\/span><\/p><\/blockquote>\n<p>(Infer here was perhaps too strong a phrasing. I meant it probabilistically.)<\/p>\n<p>In J&#8217;s words:<\/p>\n<blockquote><p><span class=\" UFICommentActorAndBody\"><span data-ft=\"{&quot;tn&quot;:&quot;K&quot;}\"><span class=\"UFICommentBody\">No, because it&#8217;s not *all* noise (obviously so in the case of sexual orientation in discordant twins). Some of it is developmental variation. Some of it is the result of pathogens\/other environmental insults.<\/span><\/span><\/span><\/p><\/blockquote>\n<p>However, he is incorrect:<\/p>\n<blockquote><p><span class=\" UFICommentActorAndBody\"> <span data-ft=\"{&quot;tn&quot;:&quot;K&quot;}\"><span class=\"UFICommentBody _1n4g\">Evidence means that the posterior probability is larger than the prior. In this case, a within MZ association rules out confounding due to A and C pathways. This is important because A confounding is probably the largest source of confounding, hence ruling it out increases the probability of all remaining options including causal connection. This being a longitudinal study (with a control too) also rules out reverse causation, further increasing the probability of forward causation.<\/p>\n<p>Your argument is ignoring the probability change. To generalize and illustrate: one cannot declare something not evidence just because it does not rule out all possible alternative interpretations. If we know that x must be one of 1, 2, &#8230; 10. Ruling out that it is 1-8 is strong evidence that it is 9, even if it is still possible that it is 10. Assuming equiprobable options, the probability increases by a factor of 5 (from 10% to 50%).<\/span><\/span><\/span><\/p><\/blockquote>\n<p>To give a concrete but simplified example. Suppose that whenever we find an association between two human variables like these, 60% of the time it is due to genetic confounding, 20% of the time it is due to shared environmental confounding, 10% of the time it is due to non-shared non-genetic confounding (this includes the developmental variation that J mentions) and 10% of the time it is causal. So, the prior probability of true causality is only 10%. However, if we then find that this relationship holds when we control for genetic and shared environmental confounding, the posterior probability of causality is now 50%. This is because only non-shared non-genetic confounding and true causality remains as possible options, both with 10% prior probability, and thus with 50% of the posterior. Thus, this represents a 5x increase in the probability. By <a href=\"https:\/\/en.wikipedia.org\/wiki\/Bayes_factor#Interpretation\">one common Bayesian standard<\/a>, this represents strong evidence.<\/p>\n<p>Back in reality, a given link between two variables will be some mix of variance pathways (e.g. 50% genetic, 30% shared environmental confounding, 20% causal), not only a single. This does not change anything substantial about the results, only makes it more complicated. (Proof of this is left to the reader!)<\/p>\n","protected":false},"excerpt":{"rendered":"<p>I found some genetically informative studies on the benefits of marriage: Does marriage inhibit antisocial behavior?: An examination of selection vs causation via a longitudinal twin design. Accounting for the physical and mental health benefits of entry into marriage: A genetically informed study of selection and causation. Marriage and Involvement in Crime: A Consideration of [&hellip;]<\/p>\n","protected":false},"author":17,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1690],"tags":[2457,2604,2603],"class_list":["post-7016","post","type-post","status-publish","format-standard","hentry","category-genetics","tag-causality","tag-jayman","tag-twin-control","entry"],"_links":{"self":[{"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/posts\/7016","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/users\/17"}],"replies":[{"embeddable":true,"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/comments?post=7016"}],"version-history":[{"count":1,"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/posts\/7016\/revisions"}],"predecessor-version":[{"id":7017,"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/posts\/7016\/revisions\/7017"}],"wp:attachment":[{"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/media?parent=7016"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/categories?post=7016"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/emilkirkegaard.dk\/en\/wp-json\/wp\/v2\/tags?post=7016"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}