Microsoft Word - 73 2017.docx   Vol  5,  No  1  (2017)   ISSN  2167-­‐8677  (online)   DOI  10.5195/d3000.2017.73           http://dentistry3000.pitt.edu     New  articles  in  this  journal  are  licensed  under  a  Creative  Commons  Attribution  4.0  United  States  License.     This  journal  is  published  by  the  University  Library  System,  University  of  Pittsburgh  as  part  of  its  D-­‐Scribe  Digital  Publishing  Program  and  is  cosponored   by  the  University  of  Pittsburgh  Press.     Behavioral  genetics  and  thumb  sucking  in  adolescents     Jessica  Hatala1      1University  of  Pi.sburgh,  School  of  Dental  Medicine Abstract   Thumb  sucking  is  a  common  habit  developed  by  children  and  begins  as  early  as  in  utero.  However,  it  becomes  prob-­‐ lema%c  when  a  child  con%nues  to  suck  their  thumb  past  the  age  of  4  years,  when  their  secondary  den,,on  is  develop-­‐ ing   and  preparing   to   erupt.   Prolonged   thumb   sucking   into   adolescence   can  have  deleterious   effects  on  dental   and   skeletal   structures   based   on   the   dura0on   and   how   frequent   the   child   engages   in   this   nonnutri0ve   sucking   habit.     Thumb  sucking  can  lead  to  various  types  of  effects  such  as  increased  overjet,  anterior  open  bite,  posterior  cross  bite,   maxillary  arch  constric-on,  high  palatal  vault,  and  Class   II  malocclusion.  This  paper  presents  a  case  study,  which  fo-­‐ cuses  on  chronic   thumb  sucking   in  a   family,   the  dental  and  skeletal  changes   that   the   family  members  experienced,   and  whether  or  not  this  prolonged  behavior  can  be  gene2cally  influenced.  Based  on  this  case  study  and  the  field  of   behavioral  gene-cs  it  is  possible  that  there  can  be  a  gene)c  component  to  the  dura)on  of  thumb  sucking  into  adoles-­‐ cence.   Cita%on:   Hatala,   J.   (2017)   Behavioral   gene-cs   and   thumb   sucking   in   adolescents.   Den$stry   3000.  1:a001  doi:10.5195/d3000.2017.73   Received:    May  5,  2017   Accepted:    May  26,  2017   Published:    June  19,  2017   Copyright:  ©2017  Hatala,   J.  This   is   an  open  ac-­‐ cess  ar!cle   licensed  under  a  Crea!ve  Commons   A"ribu%on  Work  4.0  United  States  License.   Email:  jeh193@pi*.edu   Introduction   Nonnutritive  sucking  is  a   very  common  habit  that  occurs  in   children  and  can  persist  through   adolescence  and  sometimes  even   adulthood.    Studies  have  suggest-­‐ ed  that  70-­‐90%  of  children  take   part  in  some  form  of  nonnutritive   sucking  in  their  lifetime  [1].     Nonnutritive  sucking  can  include   thumb  and  finger  sucking  and  pac-­‐ ifier  use,  with  thumb  sucking  being   the  most  prevalent.  Evidence  has   shown  that  thumb  sucking  can   start  as  early  as  29  weeks  of  gesta-­‐ tion.  At  birth,  sucking  is  a  natural   reflex  for  newborns  as  it  enables   the  child  to  breastfeed.  However,   it  can  be  engrained  in  a  habit  that   is  very  hard  to  break  for  some   children.  Most  children  stop  suck-­‐ ing  their  thumbs  by  the  age  of  4,   however  some  children  continue   to  suck  their  thumbs  into  adoles-­‐ cence  and  sometimes  even  into   adulthood.     Even  though  thumb  suck-­‐ ing  in  infants  and  toddlers  is  nor-­‐ mal,  prolonged  thumb  sucking  has   been  shown  to  have  deleterious   effects  on  the  occlusion  of  chil-­‐ dren  and  adolescents.  Primary,   mixed,  and  secondary  dentition   can  all  be  affected.    The  effects  on   the  dentition  and  maxillofacial   structures  varies  between  individ-­‐ uals  due  to  the  frequency  and  du-­‐ ration  of  the  thumb  sucking  in  that   individual.     Thumb  sucking  can  be  a  re-­‐ sult  of  environmental  factors,  but   there  has  been  some  speculation   that  prolonged  thumb  sucking  into   adolescence  may  have  some  ge-­‐ netic  influence.  Behavioral  genet-­‐ ics  is  a  branch  of  genetics  that  fo-­‐ cuses  on  how  behavior  and  per-­‐ sonality  can  be  influenced  by   genes.  This  paper  focuses  on   thumb  sucking,  the  deleterious   effects  it  can  cause  to  the  oral  and   maxillofacial  structures,  and  how   it  may  be  associated  with  behav-­‐ ioral  genetics.     Malocclusion   Chronic  nonnutritive  suck-­‐ ing,  particularly  thumb  sucking  can   have  some  deleterious  effects  on   the  primary,  mixed,  and  perma-­‐ nent  dentitions.  Studies  have   shown  that  in  the  primary  denti-­‐ tion,  nonnutritive  sucking  habits   were  associated  with  anterior   open  bite,  increased  overjet,  and   Class  II  canine  and  molar  relation-­‐ ships  [2].  Several  studies  have  also   shown  that  prolonged  nonnutri-­‐ tive  sucking  can  cause  decreased   maxillary  arch  width  and  an  in-­‐ creased  palatal  depth.  A  cohort   study  conducted  by  Warren  et  al   [2]  consisted  of  sending  question-­‐ naires  to  mothers  regarding   whether  or  not  their  child  took   part  in  any  nonnutritive  sucking   habit.  Questionnaires  were  sent   out  at  3,  6,  9,  12,  and  20  months   Behavioral  gene-cs  and  thumb  sucking  in  adolescents     Vol  5,  No  1  (2017)        DOI  10.5195/d3000.2017.73    http://dentistry3000.pitt.edu   to  assess  nutritive  and  nonnutri-­‐ tive  behaviors.  Questionnaires   were  subsequently  sent  out  at  24   months  and  then  yearly  thereaf-­‐ ter.  Study  models  of  children  4-­‐5   years  of  age  were  then  studied  to   looks  for  any  type  of  malocclusion.   Children  who  continued  their  digit   habits  until  48  months  of  age  or   older  presented  with  more  con-­‐ stricted  maxillary  arch  widths,  in-­‐ creased  maxillary  arch  depth,  in-­‐ creased  overjet,  and  anterior  open   bite  compared  to  children  with  a   shorter  habit  duration  or  those   who  had  no  habit  at  all.     Studies  focusing  on  the   mixed  dentition  are  less  common   than  the  primary  dentition  be-­‐ cause  children  should  normally   stop  any  form  of  nonnutritive   sucking  before  the  age  of  4.  How-­‐ ever,  the  few  studies  that  were   conducted  show  that  there  is  an   increase  in  malocclusion  in  those   who  continue  to  suck  their  thumb   into  their  mixed  dentition  stage.   Common  forms  of  malocclusion   that  are  often  seen  in  the  mixed   dentition  stage  include  anterior   open  bite,  posterior  cross  bite,  bi-­‐ lateral  class  II  molar  relationship,   and  overjet  >4  mm  [3].  In  a  longi-­‐ tudinal  study  conducted  by  War-­‐ ren  et  al.  [3],  dental  examinations   were  conducted  on  630  children,   and  524  sufficient  study  models   were  obtained  in  order  to  evaluate   the  effects  of  chronic  nonnutritive   sucking  habits  on  the  mixed  denti-­‐ tion.  The  children  in  the  study   were  7-­‐11  years  of  age,  with  al-­‐ most  90%  of  those  children  being   8  or  9  years  old.  Fifty-­‐five  percent   of  the  children  had  some  sort  of   malocclusion.  The  study  showed   that  for  anterior  open  bite  and   posterior  cross  bite,  the  preva-­‐ lence  increased  significantly  with  a   longer  duration  of  digit  sucking.  In   addition,  there  was  a  significant   increase  in  the  prevalence  of  a   class  II  type  malocclusion.  The   same  dental  and  skeletal  manifes-­‐ tations  can  then  persist  into  the   permanent  dentition  phase.   Behavioral  Genetics     It  has  been  speculated  that   human  behavior  and  personality   are  influenced  by  our  genetic   makeup.    The  study  of  behavioral   genetics  has  been  a  topic  of  inter-­‐ est  among  many  researchers.  In   the  case  of  thumb  sucking,  genes   can  be  influencing  the  behavior  of   children  and  can  cause  some  chil-­‐ dren  to  suck  their  thumbs  for  a   prolonged  period  of  time.  Children   usually  suck  their  thumbs  when   they  are  bored,  stressed,  anxious,   or  nervous.  According  to  Lubitz   [4],  children  suck  their  thumbs  due   to  developmental  problems  and   environmental  disturbances  and   that  thumb  sucking  is  a  way  of  re-­‐ laxation  for  many  children.  Our   genetic  composition  can  make   some  people  more  susceptible  to   these  emotions  than  others  and   can  lead  to  differences  in  behav-­‐ ior.     Children  usually  stop  suck-­‐ ing  their  thumbs  by  the  age  of  4   years.  However,  a  child  can  con-­‐ tinue  to  suck  their  thumbs  into   adolescence.  Adolescence  is  a  de-­‐ velopmental  period  that  marks   many  changes  in  physical  growth,   maturation,  and  psychological  de-­‐ velopment  [5].  The  children  who   do  suck  their  thumbs  past  the  age   of  4  years  usually  stop  sucking   their  thumbs  during  adolescence.   Rarely,  do  people  suck  their   thumbs  into  adulthood.  It  is  possi-­‐ ble  that  people  stop  sucking  their   thumbs  during  adolescence  be-­‐ cause  they  are  transitioning  into   being  their  own  person.  It  is  possi-­‐ ble  that  behavioral  genetics  is   causing  a  switch  in  the  decision-­‐ making  process  in  those  individu-­‐ als.  A  study  by  Dick  et  al  [5]  focus-­‐ es  on  adolescents  and  how  their   genes  can  affect  their  behavior   and  decision  making  processes.   They  looked  at  literature  related   to  alcohol  use  in  adolescents  and   then  concluded  that  there  is  a  ge-­‐ netic  association  with  their  deci-­‐ sion-­‐making  process.  They  believe   during  that  period  in  their  lives,   adolescents  are  becoming  their   own  person  and  are  more  likely  to   experiment  with  different  things   like  alcohol  use  and  cigarette  use.   This  study  isn’t  related  to  thumb   sucking,  but  it  can  be  used  to   show  that  adolescence  is  a  very   dynamic  period  where  individuals   are  trying  to  become  their  own   person  and  change  their  behav-­‐ iors.  That  could  be  a  possible  ex-­‐ planation  why  people  who  suck   Behavioral  gene-cs  and  thumb  sucking  in  adolescents     Vol  5,  No  1  (2017)        DOI  10.5195/d3000.2017.73    http://dentistry3000.pitt.edu   their  thumbs  for  a  prolonged  peri-­‐ od  of  time  discontinue  sucking   their  thumb  during  adolescence.     Most  of  the  studies  that   looked  at  behavior  and  genetics   consisted  of  twin  study  models.   These  models  allowed  researchers   to  determine  whether  genetics  is   responsible  for  the  behavior  or  if  it   is  mainly  the  environment  that  is   influencing  the  behavior.  In  these   studies,  if  dizygotic  twin  behavior   is  very  similar  to  monozygotic  twin   behavior,  their  environment  could   play  a  larger  role  in  the  develop-­‐ ment  of  that  particular  behavior   [5].  On  the  other  hand,  if  a  behav-­‐ ior  is  seen  more  in  monozygotic   twins  than  dizygotic  twins,  genet-­‐ ics  is  the  most  likely  explanation   since  they  have  the  same  DNA.  A   Japanese  Twin  study  by  Ooki  et  al   [6]  evaluated  the  association  of   genetics  with  thumb  sucking  and   nail  biting  in  monozygotic  and   dizygotic  twins.    The  results  of  this   study  have  shown  a  greater  preva-­‐ lence  of  thumb  sucking  in   monozygotic  twins  than  dizygotic   twins,  suggesting  that  there  is   most  likely  a  genetic  component   to  thumb  sucking.   Case  Study   A  family  where  chronic   thumb  sucking  was  common  was   studied  to  show  if  there  is  a  genet-­‐ ic  component  for  thumb  sucking  in   families.  Chronic  thumb  sucking  in   this  study  was  defined  by  an  indi-­‐ vidual  sucking  their  past  the  age  of   4  years  old.  The  study  was  per-­‐ formed  by  interviewing  the  family   members  that  were  chronic   thumb  suckers  and  observing  their   dentition  and  skeletal  orientation.   The  family  members  were  asked   what  age  did  they  discontinue   sucking  their  thumb,  whether  or   not  they  had  undergone  ortho-­‐ dontic  therapy,  and  what  made   them  discontinue  this  habit.  In  ad-­‐ dition,  the  dental  and  skeletal  re-­‐ lationship  was  observed  to  deter-­‐ mine  whether  they  had  any  sort  of   malocclusion.     The  family  members  who  chroni-­‐ cally  sucked  their  thumb  included   the  proband  in  the  third  genera-­‐ tion,  a  paternal  aunt  in  the  second   generation,  and  a  paternal  first   cousin  in  the  third  generation.  On-­‐ ly  females  were  shown  to  suck   their  thumb  for  a  prolonged  peri-­‐ od  of  time.  Two  of  the  three  rela-­‐ tives  presented  with  signs  of  mal-­‐ occlusion  or  changes  to  their  skel-­‐ etal  structures.         The  proband  is  a  23  year-­‐ old  Caucasian  female.  She  discon-­‐ tinued  sucking  her  thumb  at  the   age  of  12  years.  The  patient  cur-­‐ rently  presents  with  a  Class  I  oc-­‐ clusion  and  straight  profile.  The   proband’s  occlusion  is  normal  and   ideal.  However,  the  proband  had   undergone  orthodontic  treatment   at  the  age  of  13  years  to  correct   her  crowding.  Before  orthodontic   treatment,  the  proband  had  in-­‐ creased  maxillary  overjet.  The   proband  currently  has  a  very  high   palatal  vault,  which  is  a  common   finding  in  individuals  with  a  chron-­‐ ic  history  of  thumb  sucking  (Figure   1).  Force  of  the  thumb  onto  the   palate  during  growth  and  devel-­‐ opment  causes  the  maxillary  vault   to  increase  in  depth.  The  proband   stopped  sucking  her  thumb  at  the   age  of  12  years  due  to  peer  ac-­‐ ceptance.  She  was  going  into  mid-­‐ dle  school  and  did  not  want  any-­‐ one  to  see  her  and  make  fun  of   her.  Also,  she  was  planned  to  start   orthodontic  treatment  in  the  next   year.       The  proband’s  paternal   aunt  is  a  57  year-­‐old  Caucasian   female.  She  stopped  sucking  her   thumb  at  the  age  of  15  years.  The   Behavioral  gene-cs  and  thumb  sucking  in  adolescents     Vol  5,  No  1  (2017)        DOI  10.5195/d3000.2017.73    http://dentistry3000.pitt.edu   subject  presents  with  a  unilateral   posterior  cross  bite  on  her  left  side   with  her  mandibular  premolars   and  molars  positioned  in  front  of   her  maxillary  premolars  and  mo-­‐ lars  (Figure  2).  The  subject  also   presents  with  a  very  constricted   and  high  maxillary  palatal  vault.  In   addition,  the  subject  presents  with   an  Angle’s  class  II  malocclusion,   with  her  mandible  being  slightly   retrognathic  (Figure  3).  The  class  II   malocclusion  is  apparent  when   you  observe  the  position  of  her   canines  as  seen  in  the  pictures  be-­‐ low.  Her  canines  line  up  instead  of   her  mandibular  canine  being   slightly  ahead  of  the  maxillary  ca-­‐ nine  as  seen  in  an  ideal  Class  I  oc-­‐ clusion.  The  subject  never  re-­‐ ceived  any  orthodontic  treatment   to  correct  her  malocclusion.    She   discontinued  sucking  her  thumb  at   the  age  of  15  because  she  didn’t   want  people  to  make  fun  of  her.   She  was  able  to  break  this  long-­‐ term  habit  with  the  use  of  a  pala-­‐ tal  appliance.   The  next  subject  in  the   study  is  the  proband’s  first  cousin,   a  17  year-­‐old  Caucasian  female.   She  stopped  sucking  her  thumb  at   the  age  of  10  years.  She  presents   with  no  forms  of  dental  and  skele-­‐ tal  malocclusion.  She  had  never   had  an  orthodontic  treatment  per-­‐ formed  on  her.  This  is  most  likely   because  she  did  not  suck  her   thumb  as  long  as  the  other  sub-­‐ jects  in  the  study.  The  subject   stated  that  she  stopped  sucking   her  thumb  for  social  acceptance   and  so  people  would  not  make  fun   of  her.       By  looking  at  the  pedigree   on  the  next  page  and  seeing  that   three  people  on  one  side  of  the   family  all  sucked  their  thumbs  into   adolescence,  there  can  be  possible   genetic  component  to  this  distri-­‐ bution  of  traits  in  a  family  (Figure   4).    The  field  of  genetics  that  is   most  probable  for  influencing  this   behavior  is  behavioral  genetics.   There  is  very  little  research  on   chronic  thumb  sucking  and  behav-­‐ ioral  genetics.  However,  it  can  be  a   possible  explanation  for  the  preva-­‐ lence  of  this  habit  in  families.      The   most  likely  mode  of  inheritance   for  this  trait  is  a  multifactorial   mode  of  inheritance  where  both   genes  and  environment  play  a   role.  The  genes  involved  are  most   likely  genes  that  affect  our  per-­‐ sonality  and  decision  making.   Children  often  suck  their  thumbs   in  the  presence  of  stress,  anxiety,   and  nervousness.  It  is  possible  that   the  subjects  in  the  study  had  an   increased  level  of  stress  which   could  be  influenced  by  genetics.   The  family  members  who  had  this   habit  all  stopped  sucking  their   thumbs  during  adolescence.  Ado-­‐ lescence  is  a  transitional  period   where  children  are  starting  to   make  their  own  decisions  and  try-­‐ ing  to  become  more  independent.   In  all  three  cases,  the  subjects   wanted  to  stop  sucking  their   thumbs  for  peer  acceptance.  Per-­‐ haps,  behavioral  genes  are  affect-­‐ ing  the  decision  making  process,   and  that  is  why  the  family  mem-­‐ bers  discontinued  sucking  their   thumbs  during  adolescence.   Conclusion   Nonnutritive  sucking  is  a   common  phenomenon  that  occurs   in  young  children,  but  when  the   habit  becomes  engrained,  the  in-­‐ dividual  can  continue  to  suck  their   thumb  into  adolescence  and   sometimes  even  into  adulthood.   There  are  many  deleterious  ef-­‐ fects  that  can  result  from  the  forc-­‐ es  that  are  applied  by  the  thumb   onto  the  oral  structures.  The  dura-­‐ tion  and  frequency  of  the  sucking   influences  how  severe  these  ef-­‐ fects  can  be  on  the  dental  and   skeletal  structures.  Thumb  sucking   is  a  very  difficult  topic  to  study.     Very  little  research  has  been  per-­‐ formed  on  whether  or  not  there  is   genetic  component  to  chronic   thumb  sucking.  However,  there  is   some  speculation  that  there  can   Behavioral  gene-cs  and  thumb  sucking  in  adolescents     Vol  5,  No  1  (2017)        DOI  10.5195/d3000.2017.73    http://dentistry3000.pitt.edu   be  a  behavioral  genetic  compo-­‐ nent  related  to  prolonged  nonnu-­‐ tritive  sucking.  Often  children  who   suck  their  thumbs  are  nervous,   anxious,  or  bored.  Studies  have   shown  that  there  are  some  behav-­‐ ioral  genes  that  could  be  influenc-­‐ ing  those  emotions  and  subse-­‐ quent  behaviors.  The  case  study   presented  in  this  paper,  shows   that  thumb  sucking  can  possibly   have  a  genetic  component.  Three   individuals  on  one  side  of  a  family   all  sucked  their  thumb  into  ado-­‐ lescence.  Adolescence  is  a  transi-­‐ tional  period  with  many  develop-­‐ mental  changes  that  often  affect   the  individual’s  personality,  emo-­‐ tions,  and  decision  making.  It  is   believed  that  genes  can  affect  all   of  these  changes  throughout  ado-­‐ lescence.  It  can  be  hypothesized   that  the  behavioral  genes  play  a   role  in  the  prolonged  habit  of   thumb  sucking  and  then  the  sub-­‐ sequent  discontinuation  of  the   habit  in  adolescence.  However,  it   is  still  uncertain  whether  this  is   true  and  more  research  needs  to   be  conducted  to  obtain  more  in-­‐ formation  on  this  possible  associa-­‐ tion.     References   1. 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