Psychological studies have shown that personality differences can be reduced to a few general traits. One group of studies (the lexical approach) analyzed the way in which ordinary people talk about personality. Careful analysis of those personality adjectives found five main global traits or dimensions, which were called the Big Five [2]. There is growing evidence that people in diverse cultures, using very different languages, view individual differences in personality traits in ways similar to the Big Five.

Personality researchers have investigated this five-factor personality structure, and most have used the NEO-PI-R developed by Costa and McCrae [3]. In 2005, McCrae and Terracciano [4] conducted a crosscultural study in which researchers from 50 cultures (our research group was one of them) took part, with a total of 11,985 subjects. The personality traits were assessed by the personality questionnaire NEO-PI-R (R form), which measures the factors neuroticism, extraversion, openness to experience, agreeableness, and conscientiousness. Neuroticism is the susceptibility to experience emotional instability and strong negative emotions, such as anxiety, anger, sadness, frustration, or hostility, to be easily worried, and to have unrealistic thoughts and ideas; its opposite is emotional stability. Extraversion is the tendency to show behaviors typical of a sociable, energetic, enthusiastic person, who is active, talkative, optimistic, and fun-loving. The opposite is introversion, a tendency to be aloof and reserved. Openness is the tendency to feel curiosity, to show open and imaginative behaviors, and to be interested in new ideas and unconventional values; conventional, down-to-earth people would be at the other end of the trait spectrum. Agreeableness involves a tendency to behave in a kind and pleasant way to others and to be compassionate, empathetic, soft-hearted, trusting, and attentive, just the opposite of the other pole, which shows cynical, suspicious, and uncooperative attitudes. A person with a high level of conscientiousness is likely to be organized, a hard worker, punctual, an achiever, and persistent, while a low level of conscientiousness involves being aimless, unreliable, lazy, and negligent.

Besides the general score for the trait, the NEO-PI-R also offers scores in specific aspects, which are called facets. Every trait is composed of six facets. In the study by McCrae et al., the five-factor structure emerged not only in the combined sample, but also in the great majority of cultures, showing that the same personality traits appear in very different environments. Regarding gender differences, considering the whole sample, it was found that women scored higher than men in the big five dimensions: they were more neurotic, extravert, open, agreeable, and conscientious than men. The differences were more pronounced in the traits of neuroticism and agreeableness. In relation to facets, men were found to score higher than women in assertiveness, sensation-seeking, openness to ideas, and competence, while women exceeded men in anxiety, vulnerability, and openness to feelings and sensitivity to others.

Some female/male differences were modulated by age. This was the case of achievement motivation: in older people, men were higher achievers than women, but in younger subjects female scores in achievement motivation were higher than men’s, perhaps revealing a generational effect of an increase in women’s professional aspirations all over the world. Other male/female differences were modulated by the gender of the person who made the assessment. Thus, women (scoring higher in agreeableness) offered more positive assessments of others, especially when they assessed other women (i.e., they described them as scoring lower in neuroticism and higher in agreeableness).

The above differences, obtained with the combined sample of 50 countries, were also obtained when the analyses were carried out in the specific countries that participated in the study. Thus, it was found that differences between men and women were systematic and consistent with social stereotypes. They appeared despite variations in culture, age, and methods (self-report/other reported data). Therefore, as far as personality traits are concerned, it can be said that gender differences seem to be universal. However, these differences were quite small (0.5 standard deviation). Recently, Vianello and coworkers [5] found even smaller differences using implicit measures of traits in a large sample of more than 14,000 participants.

Different studies have tried to examine the relation between those five factors and mental health, psychological adjustment, and well-being. In general, the most predictive factor is neuroticism, which shows a systematic positive relation to maladjustment, a low level of well-being, and poor mental health. Neuroticism predisposes individuals to a wide range of psychiatric disorders: generalized anxiety disorder, panic disorder, phobias, major depression, dysthymic disorder, and borderline personality disorder are diagnosed more often in women than in men. Extraversion, on the contrary, emerges as a protective factor, being consistently associated with a higher level of well-being and good mental health. The openness factor is basically unrelated to the above variables, although specific facets may be related to maladjustment, and both agreeableness and conscientiousness show positive connections, although generally quite small, with well-being and adjustment.

Feingold [6] conducted four meta-analyses to examine gender differences in personality in the literature (1958–1992) and in normative data for well-known personality inventories (1940–1992). The results are consistent with those of the Big Five studies: men were found to be more assertive and had slightly higher self-esteem than women. Women scored higher than men in extraversion, anxiety, trust, and, especially, tender-mindedness. Gender differences in personality traits were generally constant across ages, years of data collection, educational levels, and nations.

In a much smaller study, Carrillo et al. [7] asked participants (half male, half female) to fill in the NEO-PI(R) and the Beck Depression Inventory. We were interested in the role that one of the Big Five factors, the trait of openness to experience, played in the development of symptoms of depression. It was found that one specific facet of the trait, openness to actions, was negatively related to experiencing symptoms of depression (it appeared to be adaptive), while the facet of openness to fantasy functioned as a predictor of dysphoric symptoms in the case of women, who scored higher in this facet.

It is well known that across the lifespan women are twice as likely as men to suffer from depression. Carrillo et al. [8] found a cluster of personality aspects that were related to the probability of having symptoms of depression, most of them related to experiencing highly negative emotions. The authors attribute this result to women having learned more negative emotional responses to life stimuli than men.

A very different personality trait that has received attention and shows differences between men and women is Snyder’s self-monitoring construct, a less comprehensive trait than the Big Five categories, which belongs to a long tradition in psychology, the dramaturgical model [9, 10]. According to this, we behave the way others expect us to, we are alert to subtle cues in our social environment, and, in general, we engage in self-presentation. Research has examined interpersonal differences in the degree in which persons are able to control their expressive behavior, the emotional, non-verbal cues emitted, and the adequacy of their representations in different contexts. All of this is encompassed in the self-monitoring construct developed by Snyder in 1974. Typically high self-monitors are greatly concerned with the situational and the interpersonal appropriateness of their social behavior, are particularly sensitive to the expression and self-presentation of relevant others in social situations and use this as a guideline for regulating and controlling their own self-presentation [11]. Low self-monitors, on the contrary, are frequently guided by internal aspects, are less concerned with the social environment surrounding them, and value more the congruence among what they feel, think, and do. Low self-monitors, in turn, appear to have a lower repertoire of self-presentation skills and lower expressive control than high self-monitors, who show a rich range of skills with which to present themselves in society and a good ability to control their performance.

Many studies have been conducted to explore the consequences of these two interpersonal orientations. Data show that both the relation attitude–behavior (the congruence between what we believe and what we do) and the consistency across situations are mediated by self-monitoring: congruence is higher for the low self-monitor, and so is behavioral consistency. Executives, business managers, and salesmen, among others, are used to scoring highly in self-monitoring; accountants or researchers tend to score lower. As readers may have guessed, men frequently have higher scores on the self-monitoring scale than women (see Rojo and Carrillo [12]). One problem with high self-monitors is that usually they do not communicate their internal states; the problem with low self-monitors is their dependency on their internal states and the poor control they have over them when negative. Several studies have found a significant relation between self-monitoring and psychological maladjustment, basically social anxiety and negative emotions (see Sanz and Graña [13]). There are reasons to expect that those subjects with high coherence and high situational consistency would be especially vulnerable to developing depressive symptoms in the presence of stressful events. Results consistent with this hypothesis have been found [13].

A very relevant part of research on self-monitoring concerns the social worlds of the two groups. Results suggest two different orientations toward friendship and sexuality: friends of high self-monitors are chosen “to do activities”, while low self-monitors have “friends for everything” [14]. In terms of sexuality, the first group shows “unrestricted” attitudes and values toward sexuality, while low self-monitors have relationships that are more monogamous and faithful [15]. In a replication of the latter study, Avia et al. [16] assumed that sexual orientation would be explained in part by gender. We considered that the social/interpersonal words, as well as the sexual orientation of men and women are often different, due in part to the role structure in western culture. Our hypothesis was that a restricted sexual orientation, defined by the need to have close emotional links and commitment within the sexual relation would be related to being a female, being of a lower age, and having a low score in self-monitoring. Results were consistent with our hypotheses, gender being the most discriminative variable.

To conclude this section, in many studies differences between women and men are found at the trait level, but these are generally small. The largest differences that have been found are in the domain of motor performance, some measures of sexuality (masturbation and attitudes toward uncommitted relationships), and physical aggression [17]. The cause of these differences is not easy to explain. Costa and MacCrae have a clearly biological viewpoint, and results in psychological literature are consistent with this. Other researchers, however, are more cautious: “Drawing conclusions about differences between men and women is a very tricky matter. Even if one finds such differences, it is hard to interpret them. Men and women differ biologically, but also socially. They often develop within societies which do or treat men and women unequally. Gender differences may be socially constructed rather than being biologically caused” [18]. In fact, some studies show that personality traits can change according to historical conditions. In one longitudinal study, Twenge [19] analyzed the mean levels of anxiety and neuroticism from 1950 to 1990, and found that anxiety increased significantly throughout this period. The authors conclude that cultural and historical changes produce parallel changes in personality. McCrae and Costa [20] recognized that a number of results suggest an environmental influence on traits: in women, the experience of divorce was related to decreased dominance and increased extraversion, while in Chinese undergraduates, openness and agreeableness increased when they lived in Canada.

Personality traits, although important, are not the only way to examine human personality. Personality traits are considered basic tendencies that determine recurring patterns of acting and feeling, but other concepts have also been used [20]. Characteristic adaptations are the particular forms in which people adapt to the environment. They are supposed to result from the interaction between basic tendencies and specific external influences (environment, situation, culture, etc.). While basic tendencies are assumed to be strongly determined by heredity, characteristic adaptations are culturally conditioned phenomena. Many personality psychologists have considered personality to be the particular way in which the person adapts to the environment, and have used concepts such as personal projects [21], personal strivings [22] or personal narratives [23] to capture relevant psychological differences. This level is related to processes, rather than structures. As has been said, the focus here is on “doing,” while structural models are based on “having” [24]. In an influential paper, Dan McAdams asked readers a very deep question: “What do you know when you know a person?” [25]. Most of us would think that to say that we know somebody, we would need to know his/her interests, values, preferences, and motives; what would make this person happy or sad, what would be the type of behaviors that, as far as our comprehension goes, he/she would never do, and what others would be expected instead. Clearly, traits do not give us that information. Personality, at this level, is related to goals, plans, and particular ways of talking about ourselves.