Lecture 2 Physical development 

Part 1 

Two times in the life where we grow a lot: two first years in your life and the years during early puberty development. The adolescent is now old enough to see these changes.  

Pubertal development 

• Puberty: the transitional process during which the primary sex characteristics (e.g, testes, ovaries) and secondary sex characteristics (breasts, pubic hair) mature resulting in the capacity to reproduce. 

• The changes that occur at puberty have their origins prenatally. 

• Gonads develop during fetal growth 

• Fetuses start off with a female design 

• In males androgens are secreted by the gonads initiating the process that results in male sex organs and has organizing influences on the developing brain, specifically the hypothalamic-pituitary-gonadal (HPG) axis. 

Five areas of change 

• Maturation of reproductive organs, secondary sex characteristics 

• Nervous and endocrine systems 

• Skeletal growth 

• Body composition & distribution of fat and muscle 

• Circulatory and respiratory systems 

People don't go through it at the same age. Picture of three 14 years old.  

• Three different ways of speaking about puberty: 

• Pubertal timing: early, on-time, late 

• Pubertal status: stage 

• Where are you along the process? 

1. Really no development 

2. Start to see changes 

3. Advanced change 

4. More advanced 

5. Adult body status 

• Pubertal tempo: rate 

• Tempo of pubertal development: how fast do you go through these processes? 

• Early/late puberty > teasing: what does this with your psychological wellbeing? 

• The three different ways can come together: early puberty, but fast (etc.) 

Regulation of hormonal changes 

• Two major feedback systems associated with puberty 

• HPG axis hypothalamic-pituitary-gonadal axis 

• HPA axis hypothalamic-pituitary-adrenal 

• Both use negative feedback to determine the amount of circulating hormones. 

• Two relatively independent processes occur: 

• Adrenarche occurs when the adrenal glands begin producing androgens in both boys and girls, the HPA axis is activated. 

• Androgens > muscle tissue, pubic & axillary hair, skin changes & acne 

• Gonarche occurs when the HPG axis is reactivated with large increases in gonadotropins, lutenizing hormones (LH) and follicle stimulating hormones (FSH) which are all related to development of the testes and ovaries etc. 

•  

• For girls: HPG and HPA processes are independent > there can be differences in timing > visible differences. 

Keep in mind: puberty linked with hypothalamus and surrounding limbic system 

• Limbic system: emotionality, reward system 

• Prefrontal cortex is also developing self-regulation, control. This happens relatively independently of puberty changes. 

• Imbalanced model: control parts is not fully mature, while ‘excited’ parts will be active during puberty. 

Hormones 

• Testosterone 

• Increases 10-20 times in males, slightly in females 

• Stimulates muscle growth 

• > differences in body. In sports: females can take drugs to achieve the same results as men. 

• Estrogen 

• Increases 8-10 times in females, slightly in males 

• Cause accumulation of fat under skin > female skin is softer than men skin. 

• Females have cyclic sex hormone (menstrual cycle) 

• Widening of the pelvic girdle occurs 

• Males maintain a constant hormone level 

Pubertal development 

• Girls mature about 2 years earlier than boys 

• Girls gain +- 15 kilos and 25 cm 

• Boys gain +- 20 kilos and 30 cm 

• Growth is opposite from prenatal, outward in: hand & feet grow before legs and arms 

Females 

• Breast development is the first physical sign for girls 

• Pubic hair growth starts shortly after the breast budding 

• Menarche: onset of menstrual cycle 

• Occurs shortly after peak rate of physical growth 

• Age of onset has decreased over the last century 

• Means is 12,7 years 

• Minimum level of body fat required 

• Athletes & anorexics often stop or have delayed menarche 

• Vaginal secretions increase both with and without connection to sexual thoughts and deeds 

• Increase in oil production in skin (acne) 

Males 

• Growth of testes is first physical sign 

• Growth of penis and accessory (internal) male organs > allows for ejaculation 

• Sperm production becomes fully established 

• Genitalia reach adult size at age 15 on average 

• First ejaculation at age 13 

• Nocturnal emissions (“wet dreams”) occur between 14-17 years 

• Deepening of the voice (+- 13,5 years) 

• Growth of pubic hair 

• Begins at same time as genital development. And is followed by facial and ancillary hair within 1-2 years. 

• Facial features become more masculine 

Changes in muscle and fat 

• Before puberty: boys=girls on levels of muscle and fat 

• Between 5 and 16 years: the number of muscle cells increase 14x in males and 10x in females 

• Strength spurt peaks at an average age of: 

• 15,3 in boys 

• 11,6 in girls 

• Motor coordination improves gradually peaks at age 18-19 

• Reaction time is fastest between the ages of 18-20 

• End of puberty muscle/fat ratio: 3:1 for boys, 5:4 for girls 

Factors influencing puberty  

• Genetic factors (nature) 

• Evolution: only monkeys and chimpanzees have the same pubertal spurts we see in humans 

• Family tendencies: the average difference in time of menarche for strangers is 29 months, for sisters 13 months, for identical twins 2,8 months 

• Diet and exercise (context) 

• Caloric needs increase 

• 25% for girls between 10 and 15 

• 90% for boys between 10 and 19 

• Peak caloric requirements 

• At age 12-15 in girls 

• 14-17 in boys 

• Severe prolonged malnutrition results in slower growth, delayed menarche, short adult stature 

• Context 

• Average age of menarche is lowest in industrialized nations with adequate health care and nutrition 

• Girls who keep their weight down (gymnastics, ballet dancers) experience later menarche and irregular menstrual periods 

• Famine induced delays 

• Father absence before puberty (particularly coupled with stressful homes) accelerates puberty in girls (earlier menarche) 

Part 2 

Factors influencing puberty 

• Secular Trend: 

• From 1840 to 1980, the average age of menarche has been falling from 17 to 12,5 worldwide 

• Trend is likely related to nutritional factors 

• Now stabilized 

What are the public health implications of these results? 

• Higher risk on breast cancer, metabolic disorder, cardiovascular diseases. Risk on developing depression. 

Secular trend in males? 

• Marshall and Tanner (1970), which provided age references for male pubertal development, the mean age for G2 stage was found to be 11,6 years in the UK. 

• These stabilized references are in contrast to the lowered median age of 9,7 year at G2 that has been reported from the American NHANES III study. 

• The stable mean age at G5 (15,3 years) in the NHANES III study is in contrast with the earlier age at G2. This would suggest that the tempo of male puberty is, in fact, decreasing.  

New evidence? 

• Mortality hump/accident hump: risk-taking and surplus mortality are signatures of the male human's early adult years.  

• Across time: over time 1815-2000, increase in mortality has declined .2 years across the decade > males are also showing a secular trend. 

How do we measure pubertal development? 

• Tanner Staging (physicians/nurses) 

• Girls: breast/pubic hair development 

• Boys: penis & testes/pubic hair development 

• 5 stages where 1= no development, 2=beginning stages and 5=adult 

• Self-report 

• Line drawings of Tanner Stages 

• Questionnaires 

• Pubertal development Questionnaire (PDQ, Petersen) 

• Age at menarche 

• Age at first wet dream 

• Age at growth spurt 

• Pubertal timing questions 

• Global visual inspection by researcher of clothed adolescent 

Tanner stages 

Pubertal Development Questionnaire 

• Taps five areas 

• Sample item: would you say that your growth in height: 

• Has not yet begun to spurt 

• Has barely started 

• Is definitely underway 

• Seems completed 

• Similar question about: skin changes, body hair 

• Males: deepening of voice, hair on face 

• Females: breast development, menstruation 

Are adolescents accurate/truthful when reporting on puberty? 

• Girls straightforward and honest 

• Boys say they're more advanced than they actually are 

Another methodological issue: puberty versus age and experience 

• In most adolescent studies: 

• Age, level of pubertal development, and social experience are correlated and difficult to disentangle (exception: endocrine disorder) 

• Requires studies designed to examine these issues: 

• Youth of the same age and social experience 

• (e.g., all 13 years old in the same grade) 

• But varying in level of pubertal maturation 

Theories of pubertal development – correlates and consequences of pubertal development 

• Early girls: only ones. On-time girls: also other peers (early girls and early boys). 

• Early girls and late boys are they the ones having the most psychologic problems? Social aspect: you don't want to be different from others.  

Theories of how puberty affects development 

• Status 

• Stressful Change Hypothesis 

• All change is stressful, so when you're going through a period of change, you're not going to do well. You are going to be more depressed, more conflict with parents, more problems. At soon as you’re done with those changes, everything will be ok.  

• Not much support for this theory 

• Timing 

• Off-time Hypothesis 

• Early and late maturing boys and girls are at risk (because different from average gender group) 

• Early maturing girls and late maturing boys are at risk (they're the ‘weird’ ones from the entire peergroup) 

• Maturation Disparity Hypothesis/Early Timing (Stage Termination) Hypothesis 

• If you're early mature, you have the physical body but you don't have the psychosocial maturity to handle the things that are put in your way because you look like an adult. 

• Ge & Natsuaki 

• Hormonal Influence Hypothesis 

• Ge & Natsuaki 

•  

• Stress responsivity, might be good or bad > increase or decrease changes of depression and other problems. 

• Gonadal hormones > neural circuitry > behavioral consequences 

How do we test these hypotheses? 

• Longitudinal study 

• 500 girls in Australia 

• Tested on behavior problems at 9, 13 and 15 years 

• Asked age at menarche at 15 years 

• Early =<12 

• On-time = 12-13 

• Late= 14 or older 

•  

• What theory does support the results? > early timing hypothesis 

Theories of how puberty affects development 

• Amplification hypotheses 

• Propose an interaction between individual or social processes and pubertal transition that increase the risk of adjustment problems 

• Individual 

• Accentuation hypothesis 

• Contextual amplification 

• Peer: deviant peer hypothesis 

• Parent: parenting practices 

• Neighborhood: contextual risk 

• School: heterosocial context 

• Problem behavior high at age 9: early pubertal timing was a stress factor that made these problems bigger. 

• You have a problem > experience a stressor > makes problem worse 

• Example 2: 

• It depends on the context if pubertal timing is going to have the greatest impact 

• Example 3: 

• How depressed the individuals were in the first wave of measurement, and then in the second wave of measurements. Girls who were low on depression in the first wave, there's no difference between early, average or late maturers in the second wave.  

• But girls who started depressed, where in the second wave more depressed.  

• Maternal depression, genetic factor: similar kind of effect. Kids with mothers who are highly depressed, those kids were much more depressed at wave two.  

• So: other factors moderate the effect of being early, on-time or late mature. 

Theories of how puberty affects development 

• Tempo 

• Maturation compression hypothesis 

• Boys: those who matured more quickly than their peers: 

• More depressive symptoms in 2 studies 

• Less depressive symptoms in NL 

• No link for internalizing in US and NL youth 

• More externalizing problems for US but not NL youth 

• Girls: no consistent link 

• Not only looking to tempo, but also to timing effects. Really mixed results.  

Part 3 Sleep 

With the increasing use of mobile technologies, we're beginning to recognize that these technologies do have devastating effects. 

Pubertal effects on sub-cortical regulatory systems 

• Changes in sleep 

• Changes in arousal 

• Circadian regulation: regular wake-sleep cycles 

• Challenge: finding a balance between the short-term demands & long-term goals 

• Fun – achievement 

• Limbic system (you want to have fun, emotionally) – pre-frontal cortex (cognitive) 

• Emotional – cognitive 

Sleep and the developing brain 

• Sleep is primary activity of brain in early development 

• Sleep is fundamental to the consolidation of learning 

• Especially when you’re in a whole new environment (first time to college) 

• Newborns (0-3 months) = 14-17 hours 

• Age 2-5 years = 12 hours – equal amounts of sleep and waking time 

• Adolescents need about 9 hours of sleep (adults 8) 

Sleep/arousal regulation 

• Some developmental changes in sleep regulation are: 

• Biological and linked to puberty 

• Others linked to social influences, habits and patterns 

• More activities outside the home, more use of smartphones 

• Interactions between these domains can lead to a negative spiral of consequences 

Increased vulnerability to sleep problems and sleep deprivation during puberty 

• Physical shifts: 

• Night-time sleep becomes lighter and more prune to disruptions 

• Daytime sleepiness increases during puberty 

• Changes in circadian system shifts sleep timing preferences to a delayed-sleep phase 

• > go to bed later and wake up later 

• Social shifts: 

• Less parental control over bedtime 

• Social interactions with peers 

• Additional after school demands: homework, sports 

• Electronic media (tv, internet, text-messaging) > exposure on blue light 

• Use of stimulants (caffeine and nicotine) > DFA (difficulty falling asleep) 

• Catch-up sleeps leads to jet lag on Mondays and the cycle continues 

What are the consequences of insufficient sleep in adolescents? 

• Missed school 

• Sleepiness (including micro-sleeps: missing whatever is going on in the classroom) 

• Negative synergy with alcohol 

• Tiredness (decreased motivation for a lot of activities, especially cognitive activities) 

• Irritability and low-frustration tolerance 

• Difficulties with self-control of attention, emotion, and behavior 

Sleep loss, unintentional injuries 

• Sleep deprivation creates impairments in attention, reaction time, judgement at levels comparable to intoxication with alcohol 

• Couple the sleep loss that is accrued by the end of the week with a late night party on friday > increases in drowsy-driving related accidents 

There needs to be balance 

• Between sleep and waking cycle 

Part 4 – adolescent brain development 

Brain development 

• Black box: we didn't know what changed during adolescence 

Brain development in adolescence: four general principles 

1. Adolescence is a period of heightened brain plasticity 

2. Brain maturation continues until a later age than previously believed 

3. Different systems mature at different points in time and at different rates 

4. The different developmental timetables of different regions create unique characteristics of adolescence 

Adolescent brain development 

• Prefrontal cortex and limbic system and connections between them 

•  

• Two main systems 

• Prefrontal cortex: in the front of the brain, frontal lobe, and is implicated in a lot of complex behavior: planning, decision-making, moderating, social behavior, cognitive control 

• Limbic system: amygdala, hippocampus etc.  

Jayne Blakemore's research 

• Through MRI more information about the brain. FMRI: functional MRI.  

• Prefrontal cortex: changes in adolescence 

• Gray matter: peaks in adolescence > cellbodys and connections between cells > synaptic pruning: synapses that aren't used, go away. 

• Part of the prefrontal cortex, medial prefrontal cortex, is more active in adolescence than it is in adults. This part decreases during adolescence.  

• Taking perspective from others. In adolescence more errors than in adults. 

• Limbic system: rewarding feeling to risk-taking 

Graphic: different parts at different times 

Brain maturation in adolescence 

• Synaptic pruning: taking away certain connections in the brain 

• Myelination: ncrease in the support structures around the axons 

• > resulting in improved brain function 

• Increased efficiency of local computations 

• Increased speed of neuronal transmission 

Some functions of the prefrontal cortex 

• Deliberate thinking 

• Logical reasoning 

• Planning ahead 

• Weighing costs and benefits 

• Regulating impulses 

Blakemore: medial prefrontal cortex use decreases during adolescence for social tasks 

• More errors compared to adults on these tasks too 

Some implications of immaturity in the prefrontal cortex 

• Adolescents are: 

• Less likely than adults to consider the future consequences of their actions 

• Less likely than adults to plan ahead 

• Less able than adults to control their impulses 

• Less able to simultaneously consider the risk and rewards of a decision 

Some functions of the limbic system 

• Experience of reward and punishment 

• Processing emotions 

• Associating emotions with memories 

• Processing social information 

Some implications of heightened limbic system arousal 

• Adolescents 

• Experience: higher “highs” and lower “lows” 

• Are especially sensitive to: 

• Emotional cues 

• Social information 

• Rewards 

Young adolescents show stronger preference for immediate reward 

• How do we know this? 

• Delay discounting task 

• Would you rather have $200 today or $1000 in six months? 

• The farther something is in time, the less reward it has (the more it is discounted) 

• Continue until indifference point is reached 

• Lower amount accepted short-term indicates stronger need for short-term gratification 

Age differences in delay discounting 

• Young adolescents show stronger preference for immediate reward 

How does this preference for immediate rewards relate to brain development? 

• As an adolescent, your brain is really good at seeking out new experiences, recognizing social and emotional information > brain is responsive to reward and emotion.  

• How does the teenage brain make decisions? 

• Prefrontal cortex: part of the brain that helps you think about potential consequences of actions.  

• Striatum: key component of reward system. You see something that is rewarding > striatum is responsive 

• FMRI: you can take a snapshot of the brain in motion. See how your brain is active.  

• Difference between teens and adults in how they liked the ‘sugar water’> what's the reason for this difference? 

• Striatum reacts on the water, sees it at a reward. 

• Difference: how excitable the brain is. Teens are much more excited about the same stimulus > greater activity in the brain as reaction on the sugar. 

• Test 2 with money instead of sugar. Teenagers more responsive to money than adults, but kids less responsive than teenagers. Teenager's brain is thus special. 

• Take home points 

• The teenage brain is constantly changing, even right now, this moment 

• The teenage brain is very responsive to the environment 

• The teenage brain gets really excited about rewards, emotions and new experiences. 

Hormonal influence hypothesis (mechanisms – from Peper and Dahl) 

Risk taking and brain development 

• Heightened stimulation seeking – linked with puberal development 

• Reward sensitivity > incentive driven behavior > substance use or unsafe sex 

• Pubertal status linked with reward sensitive circuitry 

• Self regulatory systems – linked purely with age/experience not with puberty 

•  

• Gray area: explaining why adolescents take more risks than other age groups. 

Hypothesis: starting the engines without a skilled driver 

• Limbic system is starting, but there's no cognitive control system to hold it back. 

• Generally for all adolescents puberty ignites passions for adolescents before the development of neurobehavioral systems necessary for self-control and affect regulation 

• Earlier timing of puberty results in several years with a sexually-mature body and sexually-activated brain circuits 

• Yet with relatively immature neurobehavioral systems necessary for self-control and affect regulation 

• Predicts: 

• Increased risk for disorders of self-control 

• Difficulties navigating complex social-emotional situations 

Hormonal influence hypothesis 

• Pubertal hormones > neural circuitry > behavioral consequences 

• Testosterone (see slide) 

• Ventral striatum > approach-related behaviors 

• Proactive aggression 

• Risk taking 

• Sensation seeking 

• Sensitivity to rewards 

• Estradiol (see slide) 

• Ventral striatum + anterior temporal lobe > approach-related behaviors 

• Risk taking 

Summary: brain changes during adolescence 

• Prefrontal cortex gradually becomes more efficient over the course of adolescence and early adulthood 

• Maturation of executive functions 

• Limbic system goes through a temporary period of heightened arousability soon after the onset of puberty 

• Increased reward sensitivity and emotional arousal 

• Different parts of brain become more interconnected 

• Better coordination of thinking and feeling 

Conclusions 

• Adolescence is a time of heightened arousal 

• This occurs against a backdrop of immature self-control 

• This combination make the period one of heightened vulnerability but also opportunity (creativity)