Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!thumper!ulysses!andante!mit-eddie!uw-beaver!teknowledge-vaxc!wlieberm From: wlieberm@teknowledge-vaxc.UUCP Newsgroups: comp.ai Subject: Re: Twin studies (was AIList V6 #86 - Philosophy) Message-ID: <22662@teknowledge-vaxc.ARPA> Date: 20 May 88 21:35:25 GMT References: <8805201729.AA28919@decwrl.dec.com> Reply-To: wlieberm@teknowledge-vaxc.UUCP (William Lieberman) Organization: Teknowledge, Inc., Palo Alto CA Lines: 66 Posted: Fri May 20 17:35:25 1988 In article <8805201729.AA28919@decwrl.dec.com> cooper@pbsvax.dec.com writes: >> <> invalid because of a tendency for their environments to continue to >> be the same.>> > >I can't say that I am overly familiar with this area, but all the "separated >twin" studies which I have seen discussed as evidential (rather than merely >suggestive of further research) have seemingly controlled for this by >comparing the variance of the characteristic under study in identical >twins separated at birth (100% genetic similarity) against fraternal >twins separated at birth (50% genetic similarity). Is there a significant >body of studies which I am unfamiliar with, or is there some reason to >believe that the treatment of identical twins after separation is >substantially different from the treatment of fraternal twins after >separation? > > > Topher Cooper > >USENET: ...{allegra,decvax,ihnp4,ucbvax}!decwrl!pbsvax.dec.com!cooper >INTERNET: cooper%pbsvax.DEC@decwrl.dec.com > or cooper@pbsvax.dec.com Topher Cooper's remarks are well-thought out and relevant. His thoughts should be extended a little, though, I feel. At first, there would not seem to be any reason to believe the treatment of identical twins (after separation) should be substantially different from the treatment of fraternal twins (after similar separation). And there may, in fact, not exist substantive difference in treatment. But what is difficult conceptually (and therefore, in practice) to control for are phenotypically-based differences (factors, such as looks, which are observable) between, on the one hand, the set of identical twins (basically no obvious differences), and on the other hand, the set of fraternal differences (plenty of obvious, overt differences, such as in their looks - say handsome vs ugly). If the fraternal twins differ only in LOOKING different (to the adult adopting parents, etc), that fact ALONE MAY cause differential behavior TOWARD those children, chain-reacting a cause and effect cycle that winds up as being measured as "differences in intelligence (or behavior) "due to" genetic-based differences! Thus, while the difference observed within the set of fraternal twins is demonstrably due to the fact of fraternal vs identical origins, the thesis that the difference is DUE to neurologically-related differences in the nervous system is NOT thus demonstrated! All that will have been demonstrated, and I think most can agree has been demonstrated, is that the differences observed are due to SOMETHING related to genetics - but one must be very cautious, until a specific anatomical, biochemical, etc. analysis has been done on the complete developmental structure of the brain to show one way or another (which we are years away from being able to do) of drawing the conclusion that psychological factors, such as intelligence, are necessarily the sole initial CAUSE of later observed behavioral differences. In other words, until such time as it will be possible to specifically measure every aspect of TOTAL behavior, one may not conclude that a genetic difference is solely (or at all) linked to a conjectured fundamentally neurologically-based difference. Bill Lieberman