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The Michigan State University Twin Registry (or MSUTR) is a registry of twins produced by researchers at Michigan State University. The MSUTR has a lifespan perspective focused on understanding risk factors for internalizing and externalizing disorders. [1]
The principal investigators of the MSUTR are Kelly L. Klump, Ph.D. and S. Alexandra Burt, Ph.D. The MSUTR involves several independent but related projects, each of which is described below:
The MSUTR Adolescent Twin Study of Behavioral Adjustment and Development is a study of 206 same-sex male, 312 same-sex female, and 120 opposite-sex twins between the ages of 10 and 16 years old. The primary purpose of the study was to investigate genetic, environmental (e.g., teasing by peers, parent-adolescent relationships), and neurobiological (i.e., gonadal hormones) risk factors for disordered eating, antisocial behavior, mood, attention/hyperactivity problems, and temperament during early adolescence. In particular, this study aimed to understand whether these psychological characteristics are related to gonadal hormone levels during puberty, whether relationships amongst these factors are influenced by genes, and whether gene x environment interactions influence their development. [1]
The MSUTR Adult Twin Study of Personality and Behavioral Adjustment is a study of 168 same-sex male, 294 same-sex female, and 270 opposite-sex twins between the ages of 18 and 30 years old. The primary purpose of this study was to examine the extent to which genetic and environmental influences on eating pathology, ADHD, depressive symptoms, anxiety, personality characteristics, and substance use differ between men and women in young adulthood. [1]
The Michigan Twins Project (MTP) is an ongoing, population-based study of roughly 20,000 same-sex male, same-sex female, and opposite sex twins between the ages of 3 and 25 years old. The primary purpose of this study is to investigate genetic and environmental influences on a range of physical and mental health variables, as well as to compile a database of twin families in Michigan interested in participating in future twin research. [1]
The MSUTR study of Hormones and Behavior across the Menstrual Cycle is an ongoing study that will include a total of 590 same-sex female twins between the ages of 16 and 22 years old. This study aims to investigate associations between ovarian hormone levels and several psychological characteristics, including mood, personality, disordered eating, pubertal development, impulsivity and risk taking behaviors. The extent to which genes influence relationships amongst these variables will also be examined. [1]
The MSUTR Child Twin Study is an ongoing study that will ultimately result in a sample of approximately 500 same-sex female twins and 500 same-sex male twins between the ages of 6 and 10 years old. This study aims to examine the ways in which genes and risk experiences (e.g., peer pressure) interact in the development of acting-out behaviors, depressed mood, anxiety, and other problems in childhood. [1]
Estrogen or oestrogen, is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal activity: estrone (E1), estradiol (E2), and estriol (E3). Estradiol, an estrane, is the most potent and prevalent. Another estrogen called estetrol (E4) is produced only during pregnancy.
Gender dysphoria (GD) is the distress a person feels due to a mismatch between their gender identity—their personal sense of their own gender—and their sex assigned at birth. The diagnostic label gender identity disorder (GID) was used until 2013 with the release of the DSM-5. The condition was renamed to remove the stigma associated with the term disorder.
The relationship between biology and sexual orientation is a subject of research. While scientists do not know the exact cause of sexual orientation, they theorize that it is caused by a complex interplay of genetic, hormonal, and environmental influences. Hypotheses for the impact of the post-natal social environment on sexual orientation, however, are weak, especially for males.
Borderline personality disorder (BPD), also known as emotionally unstable personality disorder (EUPD) or borderline pattern personality disorder is a personality disorder characterized by a long-term pattern of unstable relationships, distorted sense of self, and strong emotional reactions. Those affected often engage in self-harm and other dangerous behavior. They may also struggle with a feeling of emptiness, fear of abandonment, and detachment from reality. Symptoms of BPD may be triggered by events considered normal to others. BPD typically begins by early adulthood and occurs across a variety of situations. Substance use disorders, depression, and eating disorders are commonly associated with BPD. Approximately 10% of people affected with the disorder die by suicide. The disorder is often stigmatized in both the media and the psychiatric field.
Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders. Twin research is considered a key tool in behavioral genetics and in content fields, from biology to psychology. Twin studies are part of the broader methodology used in behavior genetics, which uses all data that are genetically informative – siblings studies, adoption studies, pedigree, etc. These studies have been used to track traits ranging from personal behavior to the presentation of severe mental illnesses such as schizophrenia.
Antisocial personality disorder is a personality disorder characterized by a long-term pattern of disregard for, or violation of, the rights of others. A weak or nonexistent conscience is often apparent, as well as a history of legal problems or impulsive and aggressive behaviour.
XY gonadal dysgenesis, also known as Swyer syndrome, is a type of hypogonadism in a person whose karyotype is 46,XY. They typically have normal female external genitalia, but functionless gonads, and if left untreated, will not experience puberty. Such gonads are typically surgically removed. The typical medical treatment is hormone replacement therapy. The syndrome was named after Gerald Swyer, an endocrinologist based in London.
Premenstrual dysphoric disorder (PMDD) is a severe and disabling form of premenstrual syndrome affecting 1.8–5.8% of menstruating women. The disorder consists of a variety of affective, behavioral and somatic symptoms that recur monthly during the luteal phase of the menstrual cycle. It affects women from their early teens up until menopause, excluding those with hypothalamic amenorrhea or during pregnancy and breastfeeding. Those with PMDD are at higher risk of suicide, with rates of suicidal thoughts 2.8 times higher, history of suicidal planning 4.15 times, and suicide attempts 3.3 times.
Sexual differentiation is the process of development of the differences between males and females from an undifferentiated zygote. Appearance of Sertoli cells in males and granulosa cells in females can be thought of as the starting point for testicular or ovarian differentiation in many species.
Human behaviour genetics is a subfield of the field of behaviour genetics that studies the role of genetic and environmental influences on human behaviour. Classically, human behavioural geneticists have studied the inheritance of behavioural traits. The field was originally focused on testing whether genetic influences were important in human behavior. It has evolved to address more complex questions such as: how important are genetic and/or environmental influences on various human behavioral traits; to what extent do the same genetic and/or environmental influences impact the overlap between human behavioral traits; how do genetic and/or environmental influences on behavior change across development; and what environmental factors moderate the importance of genetic effects on human behavior. The field is interdisciplinary, and draws from genetics, psychology, and statistics. Most recently, the field has moved into the area of statistical genetics, with many behavioral geneticists also involved in efforts to identify the specific genes involved in human behavior, and to understand how the effects associated with these genes changes across time, and in conjunction with the environment.
The hypothalamic–pituitary–gonadal axis refers to the hypothalamus, pituitary gland, and gonadal glands as if these individual endocrine glands were a single entity. Because these glands often act in concert, physiologists and endocrinologists find it convenient and descriptive to speak of them as a single system.
The heritability of autism is the proportion of differences in expression of autism that can be explained by genetic variation; if the heritability of a condition is high, then the condition is considered to be primarily genetic. Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether autism spectrum disorder (ASD) is explained more by multigene interactions or by rare mutations with major effects.
Gonadal dysgenesis is classified as any congenital developmental disorder of the reproductive system in the male or female. It is the atypical development of the gonads in an embryo, with reproductive tissue replaced with functionless, fibrous tissue, termed streak gonads. Streak gonads are a form of aplasia, resulting in hormonal failure that manifests as sexual infantism and infertility, with no initiation of puberty and secondary sex characteristics.
In the study of psychology, neuroticism has been considered a fundamental personality trait. For example, in the Big Five approach to personality trait theory, individuals with high scores for neuroticism are more likely than average to be moody and to experience such feelings as anxiety, worry, fear, anger, frustration, envy, jealousy, guilt, depressed mood, and loneliness. Such people are thought to respond worse to stressors and are more likely to interpret ordinary situations, such as minor frustrations, as appearing hopelessly difficult. They are described as often being self-conscious and shy, and tending to have trouble controlling urges and delaying gratification.
Sexual differentiation in humans is the process of development of sex differences in humans. It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination. Sexual differentiation includes development of different genitalia and the internal genital tracts and body hair plays a role in gender identification.
Gene–environment correlation is said to occur when exposure to environmental conditions depends on an individual's genotype.
Irving Isadore Gottesman was an American professor of psychology who devoted most of his career to the study of the genetics of schizophrenia. He wrote 17 books and more than 290 other publications, mostly on schizophrenia and behavioral genetics, and created the first academic program on behavioral genetics in the United States. He won awards such as the Hofheimer Prize for Research, the highest award from the American Psychiatric Association for psychiatric research. Lastly, Gottesman was a professor in the psychology department at the University of Minnesota, where he received his Ph.D.
Behavioural genetics, also referred to as behaviour genetics, is a field of scientific research that uses genetic methods to investigate the nature and origins of individual differences in behaviour. While the name "behavioural genetics" connotes a focus on genetic influences, the field broadly investigates genetic and environmental influences, using research designs that allow removal of the confounding of genes and environment. Behavioural genetics was founded as a scientific discipline by Francis Galton in the late 19th century, only to be discredited through association with eugenics movements before and during World War II. In the latter half of the 20th century, the field saw renewed prominence with research on inheritance of behaviour and mental illness in humans, as well as research on genetically informative model organisms through selective breeding and crosses. In the late 20th and early 21st centuries, technological advances in molecular genetics made it possible to measure and modify the genome directly. This led to major advances in model organism research and in human studies, leading to new scientific discoveries.
Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sex organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed. Before puberty, the external sex organs, known as primary sexual characteristics, are sex characteristics that distinguish boys and girls. Puberty leads to sexual dimorphism through the development of the secondary sex characteristics, which further distinguish the sexes.
The hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult. Prenatal hormones may be seen as the primary determinant of adult sexual orientation, or a co-factor with genes, biological factors and/or environmental and social conditions.