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In evolutionary biology and evolutionary psychology, the Trivers–Willard hypothesis, formally proposed by Robert Trivers and Dan Willard in 1973, suggests that female mammals are able to adjust offspring sex ratio in response to their maternal condition. For example, it may predict greater parental investment in males by parents in "good conditions" and greater investment in females by parents in "poor conditions" (relative to parents in good condition). The reasoning for this prediction is as follows: Assume that parents have information on the sex of their offspring and can influence their survival differentially. While pressures exist to maintain sex ratios at 50%, evolution will favor local deviations from this if one sex has a likely greater reproductive payoff than is usual.

Trivers and Willard also identified a circumstance in which reproducing individuals might experience deviations from expected offspring reproductive value—namely, varying maternal condition. In polygynous species males may mate with multiple females and low-condition males will achieve fewer or no matings. Parents in relatively good condition would then be under selection for mutations causing production and investment in sons (rather than daughters), because of the increased chance of mating experienced by these good-condition sons. Mating with multiple females conveys a large reproductive benefit, whereas daughters could translate their condition into only smaller benefits. An opposite prediction holds for poor-condition parents—selection will favor production and investment in daughters, so long as daughters are likely to be mated, while sons in poor condition are likely to be out-competed by other males and end up with zero mates (i.e., those sons will be a reproductive dead end).

The hypothesis was used to explain why, for example, Red Deer mothers would produce more sons when they are in good condition, and more daughters when in poor condition. In polyandrous species where some females mate with multiple males (and others get no matings) and males mate with one/few females (i.e., "sex-role reversed" species), these predictions from the Trivers–Willard hypothesis are reversed: parents in good condition will invest in daughters in order to have a daughter that can out-compete other females to attract multiple males, whereas parents in poor condition will avoid investing in daughters who are likely to get out-competed and will instead invest in sons in order to gain at least some grandchildren.

"Condition" can be assessed in multiple ways, including body size, parasite loads, or dominance, which has also been shown in macaques (Macaca sylvanus) to affect the sex of offspring, with dominant females giving birth to more sons and non-dominant females giving birth to more daughters. Consequently, high-ranking females give birth to a higher proportion of males than those who are low-ranking.

In their original paper, Trivers and Willard were not yet aware of the biochemical mechanism for the occurrence of biased sex ratios. Eventually, however, Melissa Larson et al. (2001) proposed that a high level of circulating glucose in the mother's bloodstream may favor the survival of male blastocysts. This conclusion is based on the observed male-skewed survival rates (to expanded blastocyst stages) when bovine blastocysts were exposed to heightened levels of glucose. As blood glucose levels are highly correlated with access to high-quality food, blood glucose level may serve as a proxy for "maternal condition". Thus, heightened glucose functions as one possible biochemical mechanism for observed Trivers–Willard effects.

Wild and West published a paper describing a mathematical model built on the Trivers–Willard hypothesis that allows precise predictions of alterations in sex-ratio under different circumstances.

The nasal cycle is the unconscious alternating partial congestion and decongestion of the nasal cavities in humans and other animals. This results in greater airflow through one nostril with periodic alternation between the nostrils. It is a physiological congestion of the nasal conchae, also called the nasal turbinates (curled bony projections within the nasal cavities), due to selective activation of one half of the autonomic nervous system by the hypothalamus. It should not be confused with pathological nasal congestion.

The majority of studies on sleep creativity have shown that sleep can facilitate insightful behavior and flexible reasoning, and there are several hypotheses about the creative function of dreams. On the other hand, a few recent studies have supported a theory of creative insomnia, in which creativity is significantly correlated with sleep disturbance.

A number of studies have found that biology can be linked with political orientation. This means that biology is a possible factor in political orientation but may also mean that the ideology a person identifies with changes a person's ability to perform certain tasks. Many of the studies linking biology to politics remain controversial and unreplicated, although the overall body of evidence is growing.

Brainbow is a process by which individual neurons in the brain can be distinguished from neighboring neurons using fluorescent proteins. By randomly expressing different ratios of red, green, and blue derivatives of green fluorescent protein in individual neurons, it is possible to flag each neuron with a distinctive color. This process has been a major contribution to the field of connectomics, traditionally known as hodology, which is the study of neural connections in the brain.

The technique was originally developed in 2007 by a team led by Jeff W. Lichtman and Joshua R. Sanes, both at Harvard University. The original technique has recently been adapted for use with other model organisms including Drosophila melanogaster, Caenorhabditis elegans, and Arabidopsis thaliana.

While earlier labeling techniques allowed for the mapping of only a few neurons, this new method allows more than 100 differently mapped neurons to be simultaneously and differentially illuminated in this manner. The resulting images can be quite striking and have won awards in science photography competitions.

Compassion fatigue is a condition characterized by emotional and physical exhaustion leading to a diminished ability to empathize or feel compassion for others, often described as the negative cost of caring. It is sometimes referred to as secondary traumatic stress (STS). According to the Professional Quality of Life Scale, burnout and secondary traumatic stress are two interwoven elements of compassion fatigue.

Compassion fatigue is considered to be the result of working directly with victims of disasters, trauma, or illness, especially in the health care industry. Individuals working in other helping professions are also at risk for experiencing compassion fatigue. These include child protection workers, veterinarians, teachers, social workers, palliative care workers, journalists, police officers, firefighters, animal welfare workers, public librarians, health unit coordinators, and Student Affairs professionals. Non-professionals, such as family members and other informal caregivers of people who have a chronic illness, may also experience compassion fatigue. The term was first coined in 1992 by Carla Joinson to describe the negative impact hospital nurses were experiencing as a result of their repeated, daily exposure to patient emergencies.

People who experience compassion fatigue may exhibit a variety of symptoms including lowered concentration, numbness or feelings of helplessness, irritability, lack of self-satisfaction, withdrawal, aches and pains, or work absenteeism.

Journalism analysts argue that news media have caused widespread compassion fatigue in society by saturating newspapers and news shows with decontextualized images and stories of tragedy and suffering. This has caused the public to become desensitized or resistant to helping people who are suffering.

The Roseto effect is the phenomenon by which a close-knit community experiences a reduced rate of heart disease. The effect is named for Roseto, Pennsylvania. The Roseto effect was first noticed in 1961 when the local Roseto doctor encountered Dr. Stewart Wolf, then head of Medicine of the University of Oklahoma, and they discussed, over a couple of beers, the unusually low rate of myocardial infarction in the Italian American community of Roseto compared with other locations. Many studies followed, including a 50-year study comparing Roseto to nearby Bangor. As the original authors had predicted, as the Roseto cohort shed their Italian social structure and became more Americanized in the years following the initial study, heart disease rates increased, becoming similar to those of neighboring towns.

From 1954 to 1961, Roseto had nearly no heart attacks for the otherwise high-risk group of men 55 to 64, and men over 65 had a death rate of 1% while the national average was 2%. Widowers outnumbered widows, as well.

These statistics were at odds with a number of other factors observed in the community. They smoked unfiltered stogies, drank wine "with seeming abandon" in lieu of milk and soft drinks, skipped the Mediterranean diet in favor of meatballs and sausages fried in lard with hard and soft cheeses. The men worked in the slate quarries where they contracted illnesses from gases and dust. Roseto also had no crime, and very few applications for public assistance.

Wolf attributed Rosetans' lower heart disease rate to lower stress. "'The community,' Wolf says, 'was very cohesive. There was no keeping up with the Joneses. Houses were very close together, and everyone lived more or less alike.'" Elders were revered and incorporated into community life. Housewives were respected, and fathers ran the families.